You could try to write your own class which uses HTML::Form as a base class.
You'll need something like:

package Invalid;

use base qw(HTML::Form);
use strict; # || loose(XP);

sub new {
  my $class = ref($_[0]) || $_[0]; shift;
  my $self = $class->SUPER::new(@_);
  return(bless($self,$class));
}

sub your_overwritten_routine {
  my $self = shift;
  ... and the rest of your code ...
  return $something;
}

"True value";
[download]

This way you can still be using all your code, you'll just have to 'use Invalid' instead of 'use HTML::Form' and do a 'Invalid->new(...)' instead of 'HTML::Form->new(...)'.

If a new version of HTML::Form ever comes out - nothing 'bad' happens...

Hope this any good.

Sinister greetings.
"With tying hashes you can do everything God and Larry have forbidden" -- Johan Vromans - YAPC::Europe 2001
perldoc -q $_