A bit tidier and more reliable:

#!/usr/bin/perl -w
use strict;

use constant URL => 'openURL(%s, new-tab)';
use constant MAIL => 'mailto(%s)';

sub abs_path {
    require File::Spec;
    File::Spec->rel2abs(shift);
}

$_ = shift || '';
my $command =
    m!^(?:ftp|http|file)://! ? sprintf  URL, $_                     :
    -r($_)                   ? sprintf  URL, 'file://'.abs_path($_) :
    s!^(www\..+)!http://$1!  ? sprintf  URL, $_                     :
    s!^(ftp\..+)!ftp://$1!   ? sprintf  URL, $_                     :
    s/^mailto://             ? sprintf MAIL, $_                     :
    die "usage: $0 (filename|URL|mailto:foo\@bar.com)\n";

exec 'mozilla', -remote => sprintf($command, $_);
[download]

This checks -r early - thus, you can use it to invoke Mozilla on a directory called www.foo.bar/ without the scheme guessing getting in your way. It also handles URLs with ftp.* hosts nicely.

Makeshifts last the longest.