#!/usr/bin/perl
use WWW::Mechanize ;

#Prepopulated Information
my $fname="Test";
my $lname="User";
my $address="1234 Example Street";
my $city="New York";
my $state="New York";
my $zip="10010";

my $mech = WWW::Mechanize->new ;

$mech ->get("http://your.url/") ;
die $mech ->res->status_line unless $mech ->success ;

#Hand off to HTML::Form
my @webforms = $mech->forms();
foreach my $form (@webforms) {
    my @inputfields = $form->param;
    foreach my $inputfield (@inputfields) {
        if($inputfield =~ /(F|f)(irst)?name/) {
        $mech->set_fields( $inputfield => $fname);
        }
        if($inputfield =~ /(L|l(ast)?name/) {
        $mech->set_fields( $inputfield => $lname);
        }
        if($inputfield =~ /(A|a)ddress/) {
        $mech->set_fields( $inputfield => $address);
        }
        if($inputfield =~ /(C|c)ity//) {
        $mech->set_fields( $inputfield => $city);
        }        
        if($inputfield =~ /(S|s)tate//) {
        $mech->set_fields( $inputfield => $state);
        }
        if($inputfield =~ /(Z|z)ip//) {
        $mech->set_fields( $inputfield => $zip);
        }
        if($inputfield =~ /(P|p)hone//) {
        $mech->set_fields( $inputfield => $phone);
        }
    # Submit
    $mech ->submit ;
    die $mech ->res->status_line unless $mech ->success ;
        # If the form sends you somewhere, you can catch it :
        my $url = $mech ->response->request->uri->as_string ;
    }
    
}
[download]

That's about what I was thinking. You've got extra slashes on the end of some of your regexes, and some can be simplified, e.g.

/(S|s)tate/
[download]

can be:

/[Ss]tate/
[download]

and if you don't care about case sensitivity at all, you can use:

/(?i)state/
[download]

or

/state/i
[download]