use strict;
use warnings;
use HTML::TokeParser::Simple;

my $p = HTML::TokeParser::Simple->new("foo.html");

sub new_form_tag {
    my ($attr) = @_;

    $attr->{value} = "foo"
        if $attr->{name} eq "bar";

    return join " ", "<input", map(qq($_="$attr->{$_}"), keys %$attr),
+ ">";
}

while(my $token = $p->get_token) {
    print $token->is_start_tag('input')
        ? new_input_tag($token->return_attr)
        : $token->as_is;
}
[download]

The $attr->{value} = "foo" if $attr->{name} eq "bar" part can be as complex as desired, obviously.

Makeshifts last the longest.