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The biggest trap of all is forgetting to use warnings or use the -w
switch; see perllexwarn and perlrun. The second biggest trap is not
making your entire program runnable under use strict. The third biggest
trap is not reading the list of changes in this version of Perl; see
perldelta.
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allows you to refer to special variables (like $/) with names (like
$RS), as though they were in awk; see perlvar for details.
Semicolons are required after all simple statements in Perl (except
at the end of a block). Newline is not a statement delimiter.
Curly brackets are required on ifs and whiles.
Variables begin with ``$'', ``@'' or ``%'' in Perl.
Arrays index from 0. Likewise string positions in substr() and
index().
You have to decide whether your array has numeric or string indices.
Hash values do not spring into existence upon mere reference.
You have to decide whether you want to use string or numeric
comparisons.
Reading an input line does not split it for you. You get to split it
to an array yourself. And the split() operator has different
arguments than awk's.
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The current input line is normally in $_, not $0. It generally does
not have the newline stripped. ($0 is the name of the program
executed.) See perlvar.
$<digit> does not refer to fields--it refers to substrings matched
by the last match pattern.
The print() statement does not add field and record separators unless
you set $, and $\. You can set $OFS and $ORS if you're using
the English module.
You must open your files before you print to them.
The range operator is ``..'', not comma. The comma operator works as in
C.
The match operator is ``=~'', not ``~''. (``~'' is the one's complement
operator, as in C.)
The exponentiation operator is ``**'', not ``^''. ``^'' is the XOR
operator, as in C. (You know, one could get the feeling that awk is
basically incompatible with C.)
The concatenation operator is ``.'', not the null string. (Using the
null string would render /pat/ /pat/ unparsable, because the third slash
would be interpreted as a division operator--the tokenizer is in fact
slightly context sensitive for operators like ``/'', ``?'', and ``>''.
And in fact, ``.'' itself can be the beginning of a number.)
The next, exit, and continue keywords work differently.
The following variables work differently:
Awk Perl
ARGC scalar @ARGV (compare with $#ARGV)
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
FS (whatever you like)
NF $#Fld, or some such
NR $.
OFMT $#
OFS $,
ORS $\
RLENGTH length($&)
RS $/
RSTART length($`)
SUBSEP $;
You cannot set $RS to a pattern, only a string.
When in doubt, run the awk construct through a2p and see what it
gives you.
Cerebral C and C++ programmers should take note of the following:
Curly brackets are required on if's and while's.
You must use elsif rather than else if.
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The break and continue keywords from C become in Perl last
and next, respectively. Unlike in C, these do not work within a
do { } while construct. See perlsyn/``Loop Control''.
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There's no switch statement. (But it's easy to build one on the fly,
see perlsyn/``Basic BLOCKs and Switch Statements'')
Variables begin with ``$'', ``@'' or ``%'' in Perl.
Comments begin with ``#'', not ``/*'' or ``//''. Perl may interpret C/C++
comments as division operators, unterminated regular expressions or
the defined-or operator.
You can't take the address of anything, although a similar operator
in Perl is the backslash, which creates a reference.
ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0]
ends up in $0.
System calls such as link(), unlink(), rename(), etc. return nonzero for
success, not 0. (system(), however, returns zero for success.)
Signal handlers deal with signal names, not numbers. Use kill -l
to find their names on your system.
Sharp shell programmers should take note of the following:
The backtick operator does variable interpolation without regard to
the presence of single quotes in the command.
The backtick operator does no translation of the return value, unlike csh.
Shells (especially csh) do several levels of substitution on each
command line. Perl does substitution in only certain constructs
such as double quotes, backticks, angle brackets, and search patterns.
Shells interpret scripts a little bit at a time. Perl compiles the
entire program before executing it (except for BEGIN blocks, which
execute at compile time).
The arguments are available via @ARGV, not $1, $2, etc.
The environment is not automatically made available as separate scalar
variables.
The shell's test uses ``='', ``!='', ``<'' etc for string comparisons and ``-eq'',
``-ne'', ``-lt'' etc for numeric comparisons. This is the reverse of Perl, which
uses eq, ne, lt for string comparisons, and ==, !=< etc
for numeric comparisons.
Practicing Perl Programmers should take note of the following:
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Remember that many operations behave differently in a list
context than they do in a scalar one. See perldata for details.
Avoid barewords if you can, especially all lowercase ones.
You can't tell by just looking at it whether a bareword is
a function or a string. By using quotes on strings and
parentheses on function calls, you won't ever get them confused.
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You cannot discern from mere inspection which builtins
are unary operators (like chop() and chdir())
and which are list operators (like print() and unlink()).
(Unless prototyped, user-defined subroutines can only be list
operators, never unary ones.) See perlop and perlsub.
People have a hard time remembering that some functions
default to $_, or @ARGV, or whatever, but that others which
you might expect to do not.
The <FH> construct is not the name of the filehandle, it is a readline
operation on that handle. The data read is assigned to $_ only if the
file read is the sole condition in a while loop:
while (<FH>) { }
while (defined($_ = <FH>)) { }..
<FH>; # data discarded!
Remember not to use = when you need =~;
these two constructs are quite different:
$x = /foo/;
$x =~ /foo/;
The do {} construct isn't a real loop that you can use
loop control on.
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Use my() for local variables whenever you can get away with
it (but see perlform for where you can't).
Using local() actually gives a local value to a global
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.
If you localize an exported variable in a module, its exported value will
not change. The local name becomes an alias to a new value but the
external name is still an alias for the original.
Anything that's been fixed as a perl4 bug, removed as a perl4 feature
or deprecated as a perl4 feature with the intent to encourage usage of
some other perl5 feature.
If you find an example of a conversion trap that is not listed here,
please submit it to <perlbug@perl.org> for inclusion.
Also note that at least some of these can be caught with the
use warnings pragma or the -w switch.
Given that :: is now the preferred package delimiter, it is debatable
whether this should be classed as a bug or not.
(The older package delimiter, ' ,is used here)
The second and third arguments of splice() are now evaluated in scalar
context (as the Camel says) rather than list context.
sub sub1{return(0,2) } # return a 2-element list
sub sub2{ return(1,2,3)} # return a 3-element list
@a1 = ("a","b","c","d","e");
@a2 = splice(@a1,&sub1,&sub2);
print join(' ',@a2),"\n";
The meaning of foreach{} has changed slightly when it is iterating over a
list which is not an array. This used to assign the list to a
temporary array, but no longer does so (for efficiency). This means
that you'll now be iterating over the actual values, not over copies of
the values. Modifications to the loop variable can change the original
values.
To retain Perl4 semantics you need to assign your list
explicitly to a temporary array and then iterate over that. For
example, you might need to change
foreach $var (grep(/ab/,@list)){
to
foreach $var (@tmp = grep(/ab/,@list)){
Otherwise changing $var will clobber the values of @list. (This most often
happens when you use $_ for the loop variable, and call subroutines in
the loop that don't properly localize $_.)
split with no arguments now behaves like split ' ' (which doesn't
return an initial null field if $_ starts with whitespace), it used to
behave like split /\s+/ (which does).
Perl 4 would ignore any text which was attached to an -e switch,
always taking the code snippet from the following arg. Additionally, it
would silently accept an -e switch without a following arg. Both of
these behaviors have been fixed.
perl -e'print "attached to -e"' 'print "separate arg"'
# perl4 prints: separate arg
# perl5 prints: attached to -e
perl -e
# perl4 prints:
# perl5 dies: No code specified for -e.
In Perl 4 the return value of push was undocumented, but it was
actually the last value being pushed onto the target list. In Perl 5
the return value of push is documented, but has changed, it is the
number of elements in the resulting list.
In Perl 4, if in list context the delimiters to the first argument of
split() were ??, the result would be placed in @_ as well as
being returned. Perl 5 has more respect for your subroutine arguments.
When perl sees map { (or grep {), it has to guess whether the {
starts a BLOCK or a hash reference. If it guesses wrong, it will report
a syntax error near the } and the missing (or unexpected) comma.
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Use unary + before { on a hash reference, and unary + applied
to the first thing in a BLOCK (after {), for perl to guess right all
the time. (See perlfunc/map.)
This specific item has been deleted. It demonstrated how the auto-increment
operator would not catch when a number went over the signed int limit. Fixed
in version 5.003_04. But always be wary when using large integers.
If in doubt:
Assignment of return values from numeric equality tests
does not work in perl5 when the test evaluates to false (0).
Logical tests now return a null, instead of 0
Bitwise string ops
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When bitwise operators which can operate upon either numbers or
strings (& | ^ ~) are given only strings as arguments, perl4 would
treat the operands as bitstrings so long as the program contained a call
to the vec() function. perl5 treats the string operands as bitstrings.
(See perlop/Bitwise String Operators for more details.)
$fred = "10";
$barney = "12";
$betty = $fred & $barney;
print "$betty\n";
# Uncomment the next line to change perl4's behavior
# ($dummy) = vec("dummy", 0, 0);
# Perl4 prints:
8
# Perl5 prints:
10
# If vec() is used anywhere in the program, both print:
10
Assigning undef to a glob has no effect in Perl 5. In Perl 4
it undefines the associated scalar (but may have other side effects
including SEGVs). Perl 5 will also warn ifundef is assigned to a
typeglob. (Note that assigning undef to a typeglob is different
than calling the undef function on a typeglob (undef *foo), which
has quite a few effects.
$foo = "bar";
*foo = undef;
print $foo;
# perl4 prints:
# perl4 warns: "Use of uninitialized variable" if using -w
# perl5 prints: bar
# perl5 warns: "Undefined value assigned to typeglob" if using -w
Variable suicide behavior is more consistent under Perl 5.
Perl5 exhibits the same behavior for hashes and scalars,
that perl4 exhibits for only scalars.
The caller() function now returns a false value in a scalar context if there is no caller. This lets library files determine if they're
being required.
caller() ? (print "You rang?\n") : (print "Got a 0\n");
# perl4 errors: There is no caller
# perl5 prints: Got a 0
sprintf() is prototyped as ($;@), so its first argument is given scalar
context. Thus, if passed an array, it will probably not do what you want,
unlike Perl 4:
Perl 4 has almost the same precedence rules as Perl 5 for the operators
that they both have. Perl 4 however, seems to have had some
inconsistencies that made the behavior differ from what was documented.
LHS vs. RHS of any assignment operator. LHS is evaluated first
in perl4, second in perl5; this can affect the relationship
between side-effects in sub-expressions.
These are now semantic errors because of precedence:
@list = (1,2,3,4,5);
%map = ("a",1,"b",2,"c",3,"d",4);
$n = shift @list + 2; # first item in list plus 2
print "n is $n, ";
$m = keys %map + 2; # number of items in hash plus 2
print "m is $m\n";
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
The precedence of assignment operators is now the same as the precedence
of assignment. Perl 4 mistakenly gave them the precedence of the associated
operator. So you now must parenthesize them in expressions like
perl4 had buggy precedence for the file test operators vis-a-vis
the assignment operators. Thus, although the precedence table
for perl4 leads one to believe -e $foo .= "q" should parse as
((-e $foo) .= "q"), it actually parses as (-e ($foo .= "q")).
In perl5, the precedence is as documented.
-e $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
In perl4, keys(), each() and values() were special high-precedence operators
that operated on a single hash, but in perl5, they are regular named unary
operators. As documented, named unary operators have lower precedence
than the arithmetic and concatenation operators + - ., but the perl4
variants of these operators actually bind tighter than + - ..
Thus, for:
%foo = 1..10;
print keys %foo - 1
# perl4 prints: 4
# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
The perl4 behavior was probably more useful, ifless consistent.
m//g now attaches its state to the searched string rather than the
regular expression. (Once the scope of a block is left for the sub, the
state of the searched string is lost)
Currently, if you use the m//o qualifier on a regular expression
within an anonymous sub, all closures generated from that anonymous
sub will use the regular expression as it was compiled when it was used
the very first time in any such closure. For instance, if you say
For most builds of Perl5, this will print:
ok
not ok
not ok
build_match() will always return a sub which matches the contents of
$left and $right as they were the first time that build_match()
was called, not as they are in the current call.
Stricter parsing of variables used in regular expressions
s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
# perl4: compiles w/o error
# perl5: with Scalar found where operator expected ..., near "$opt$plus"
an added component of this example, apparently from the same script, is
the actual value of the s'd string after the substitution.
[$opt] is a character class in perl4 and an array subscript in perl5
Under perl5, m?x? matches only once, like ?x?. Under perl4, it matched
repeatedly, like /x/ or m!x!.
$test = "once";
sub match { $test =~ m?once?; }
&match();
if( &match() ) {
# m?x? matches more then once
print "perl4\n";
} else {
# m?x? matches only once
print "perl5\n";
}
The general group of Perl4-to-Perl5 traps having to do with
Signals, Sorting, and their related subroutines, as well as
general subroutine traps. Includes some OS-Specific traps.
Barewords that used to look like strings to Perl will now look like subroutine
calls if a subroutine by that name is defined before the compiler sees them.
sub SeeYa { warn"Hasta la vista, baby!" }
$SIG{'TERM'} = SeeYa;
print "SIGTERM is now $SIG{'TERM'}\n";
# perl4 prints: SIGTERM is now main'SeeYa
# perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
Under HPUX, and some other SysV OSes, one had to reset any signal handler,
within the signal handler function, each time a signal was handled with
perl4. With perl5, the reset is now done correctly. Any code relying
on the handler _not_ being reset will have to be reworked.
Since version 5.002, Perl uses sigaction() under SysV.
Under SysV OSes, seek() on a file opened to append >> now does
the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
for append, it is impossible to overwrite information already in
the file.
@ now always interpolates an array in double-quotish strings.
print "To: someone@somewhere.com\n";
# perl4 prints: To:someone@somewhere.com
# perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
# perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
Perl now sometimes evaluates arbitrary expressions inside braces that occur
within double quotes (usually when the opening brace is preceded by $
or @).
Creation of hashes on the fly with eval "EXPR" now requires either both
$'s to be protected in the specification of the hash name, or both curlies
to be protected. If both curlies are protected, the result will be compatible
with perl4 and perl5. This is a very common practice, and should be changed
to use the block form of eval{}if possible.
You also have to be careful about array and hash brackets during
interpolation.
print "$foo["
perl 4 prints: [
perl 5 prints: syntax error
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
Perl 5 is expecting to find an index or key name following the respective
brackets, as well as an ending bracket of the appropriate type. In order
to mimic the behavior of Perl 4, you must escape the bracket like so.
Perl 5 is looking for $foo{bar} which doesn't exist, but perl 4 is
happy just to expand $foo to ``baz'' by itself. Watch out for this
especially in eval's.
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail. The build of perl5
must have been linked with the same dbm/ndbm as the default for dbmopen()
to function properly without tie'ing to an extension dbm implementation.
dbmopen (%dbm, "file", undef);
print "ok\n";
# perl4 prints: ok
# perl5 prints: ok (IFF linked with -ldbm or -lndbm)
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail. The error generated
when exceeding the limit on the key/value size will cause perl5 to exit
immediately.
dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
$DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
print "YUP\n";
# perl4 prints:
dbm store returned -1, errno 28, key "trap" at - line 3.
YUP
# perl5 prints:
dbm store returned -1, errno 28, key "trap" at - line 3.
