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
write() function

format

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.