...
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
}
...