#!/usr/bin/perl -w
use strict;
use HTML::Parser;

my $p = HTML::Parser->new(    
        start_h => [\&Start, 'tagname, attr, text '],
        end_h => [sub { print  shift;  }, 'text'],
        comment_h => [sub { print  shift;  }, 'text'],
        default_h => [sub { print  shift;  }, 'text'],
    );
    
    open (IN, "<form.template") or die "Couldn't open!:$!";    # Your 
+html
    my $content = '';
    {
    local $/=undef;
    $content = <IN>;

    }

    $p->parse($content);

    close(IN) or die "Couldn't close:$!";

################################################
sub Start{ 
    my ($tag, $attr,$text) = @_; 
    

    if ($tag eq 'mytag'){
        $text = "WHATEVER YOU WANT TO REPLACE HERE.
    }

    print $text;


}
[download]

CGI.pm handles custom HTML tags just fine. You need to add the custom tag names to the list of function names imported from the module and it will synthesis functions for the new tags.

use CGI qw(:standard myscriptname);

print myscriptname({type => 'type_of_field',
                    value=> 'list,of,values'});
[download]

This prints

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

The second way is to use AxKit and custom tag libraries.

Don't get me wrong I'm all for reinventing wheels especially when it helps you to learn something but I would suggest that you take a look at things that have already done this. My personal favorite is HTML::Template

You might be able to build off of that to add your own tags or just use what's already there

Some clever or funny quote here.