See the current Perl documentation for format.
Here is our local, out-dated (pre-5.6) version:
format - declare a picture format with use by the
Declare a picture format for use by the write() function. For example:
format Something = Test: @<<<<<<<< @||||| @>>>>> $str, $%, '$' . int($num) .
$str = "widget"; $num = $cost/$quantity; $~ = 'Something'; write;
See the perlform manpage for many details and examples.