Top-of-form processing is by default handled by a format with the same name as the current filehandle with ``_TOP'' concatenated to it. It's triggered at the top of each page. See perlfunc/write.

Examples:

 # a report on the /etc/passwd file
 format STDOUT_TOP =
                         Passwd File
 Name                Login    Office   Uid   Gid Home
 ------------------------------------------------------------------
 .
 format STDOUT =
 @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
 $name,              $login,  $office,$uid,$gid, $home
 .

 # a report from a bug report form
 format STDOUT_TOP =
                         Bug Reports
 @<<<<<<<<<<<<<<<<<<<<<<<     @|||         @>>>>>>>>>>>>>>>>>>>>>>>
 $system,                      $%,         $date
 ------------------------------------------------------------------
 .
 format STDOUT =
 Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          $subject
 Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        $index,                       $description
 Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
           $priority,        $date,   $description
 From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
       $from,                         $description
 Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
              $programmer,            $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<...
                                      $description
 .

It is possible to intermix print()s with write()s on the same output channel, but you'll have to handle $- ($FORMAT_LINES_LEFT) yourself.

Format Variables

The current format name is stored in the variable $~ ($FORMAT_NAME), and the current top of form format name is in $^ ($FORMAT_TOP_NAME). The current output page number is stored in $% ($FORMAT_PAGE_NUMBER), and the number of lines on the page is in $= ($FORMAT_LINES_PER_PAGE). Whether to autoflush output on this handle is stored in $| ($OUTPUT_AUTOFLUSH). The string output before each top of page (except the first) is stored in $^L ($FORMAT_FORMFEED). These variables are set on a per-filehandle basis, so you'll need to select() into a different one to affect them:

    select((select(OUTF),
            $~ = "My_Other_Format",
            $^ = "My_Top_Format"
           )[0]);

Pretty ugly, eh? It's a common idiom though, so don't be too surprised when you see it. You can at least use a temporary variable to hold the previous filehandle: (this is a much better approach in general, because not only does legibility improve, you now have intermediary stage in the expression to single-step the debugger through):

    $ofh = select(OUTF);
    $~ = "My_Other_Format";
    $^ = "My_Top_Format";
    select($ofh);

If you use the English module, you can even read the variable names:

    use English '-no_match_vars';
    $ofh = select(OUTF);
    $FORMAT_NAME     = "My_Other_Format";
    $FORMAT_TOP_NAME = "My_Top_Format";
    select($ofh);

But you still have those funny select()s. So just use the FileHandle module. Now, you can access these special variables using lowercase method names instead:

    use FileHandle;
    format_name     OUTF "My_Other_Format";
    format_top_name OUTF "My_Top_Format";

Much better!

NOTES

Because the values line may contain arbitrary expressions (for at fields, not caret fields), you can farm out more sophisticated processing to other functions, like sprintf() or one of your own. For example:

    format Ident =
        @<<<<<<<<<<<<<<<
        &commify($n)
    .

To get a real at or caret into the field, do this:

    format Ident =
    I have an @ here.
            "@"
    .

To center a whole line of text, do something like this:

    format Ident =
    @|||||||||||||||||||||||||||||||||||||||||||||||
            "Some text line"
    .

There is no builtin way to say ``float this to the right hand side of the page, however wide it is.'' You have to specify where it goes. The truly desperate can generate their own format on the fly, based on the current number of columns, and then eval() it:

    $format  = "format STDOUT = \n"
             . '^' . '<' x $cols . "\n"
             . '$entry' . "\n"
             . "\t^" . "<" x ($cols-8) . "~~\n"
             . '$entry' . "\n"
             . ".\n";
    print $format if $Debugging;
    eval $format;
    die $@ if $@;

Which would generate a format looking something like this:

 format STDOUT =
 ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 $entry
         ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
 $entry
 .

Here's a little program that's somewhat like fmt(1):

 format =
 ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
 $_

 .

 $/ = '';
 while (<>) {
     s/\s*\n\s*/ /g;
     write;
 }

Footers

While $FORMAT_TOP_NAME contains the name of the current header format, there is no corresponding mechanism to automatically do the same thing for a footer. Not knowing how big a format is going to be until you evaluate it is one of the major problems. It's on the TODO list.

Here's one strategy: If you have a fixed-size footer, you can get footers by checking $FORMAT_LINES_LEFT before each write() and print the footer yourself if necessary.

perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlform.pod: cannot resolve L in paragraph 74.

Here's another strategy: Open a pipe to yourself, using open(MYSELF, "|-") (see perlfunc/open()) and always write() to MYSELF instead of STDOUT. Have your child process massage its STDIN to rearrange headers and footers however you like. Not very convenient, but doable.

Accessing Formatting Internals

For low-level access to the formatting mechanism. you may use formline() and access $^A (the $ACCUMULATOR variable) directly.

For example:

    $str = formline <<'END', 1,2,3;
    @<<<  @|||  @>>>
    END

    print "Wow, I just stored `$^A' in the accumulator!\n";

Or to make an swrite() subroutine, which is to write() what sprintf() is to printf(), do this:

    use Carp;
    sub swrite {
        croak "usage: swrite PICTURE ARGS" unless @_;
        my $format = shift;
        $^A = "";
        formline($format,@_);
        return $^A;
    }

    $string = swrite(<<'END', 1, 2, 3);
 Check me out
 @<<<  @|||  @>>>
 END
    print $string;

WARNINGS

The lone dot that ends a format can also prematurely end a mail message passing through a misconfigured Internet mailer (and based on experience, such misconfiguration is the rule, not the exception). So when sending format code through mail, you should indent it so that the format-ending dot is not on the left margin; this will prevent SMTP cutoff.

Lexical variables (declared with ``my'') are not visible within a format unless the format is declared within the scope of the lexical variable. (They weren't visible at all before version 5.001.)

perldoc2tree.cgi: /usr/lib/perl5/5.8.8/pod/perlform.pod: cannot resolve L in paragraph 86.

Formats are the only part of Perl that unconditionally use information from a program's locale; if a program's environment specifies an LC_NUMERIC locale, it is always used to specify the decimal point character in formatted output. Perl ignores all other aspects of locale handling unless the use locale pragma is in effect. Formatted output cannot be controlled by use locale because the pragma is tied to the block structure of the program, and, for historical reasons, formats exist outside that block structure. See perllocale for further discussion of locale handling.

Within strings that are to be displayed in a fixed length text field, each control character is substituted by a space. (But remember the special meaning of \r when using fill mode.) This is done to avoid misalignment when control characters ``disappear'' on some output media.