Normally I just say RTFM, but I understand the situation in which you find yourself. I had played with perl for some time when my father's company's web design company decided to change backend languages and wished to charge his company for an entire rewrite (gotta love companies like that; code was a nightmare, btw). Anyhow, you can* read about it in my autobiography and won't bore you with the eventual triumph of our hero. Enough about me. I offer some code to assist you with your troubles.

...
my %option_to_address = (
User1 => 'user1@domain.com', # note the single quotes
User2 => 'user2@domain.com',
User3 => 'user3@domain.com' 
);
...
# take all of the values returned for m_EmailGroups and 
# build an address list
my @addresses = map($option_to_address{$_},grep($option_to_address{$_}
+,$query->param("m_EmailGroups")));
for my $address (@addresses) {
  # setup the email stuff here
}
...
[download]

Pretty much the only things left to you are setting up %option_to_address, writing the html for them, and figuring out how to convert SendMail and SendMailBySMTP to use the addresses you're looking for. I haven't looked at them much, but I think they send the e-mail. Anywho, suggested reading: How to RTFM, Net::SMTP, map, grep, perldsc, and most importantly the camel. Also, the quality of this script is somewhat questionable. It doesn't use strict and I saw at least one place where soft references were used. I am only mentioning this so you don't adopt these practices.

Hope this helps.

* Disclaimer: actually, too busy writing perl scripts and modules to consider writing something about myself.

antirice    
The first rule of Perl club is - use Perl
The ith rule of Perl club is - follow rule i - 1 for i > 1