my @mail_groups = $q->param( 'mailgroup' );

my %group_addresses = (
     group1 => 'group1@domain.com',
     group2 => 'group2@domain.com',
     group3 => 'group3@domain.com',
);

my @mail_addresses = @group_addresses{ @mail_groups };
my $to = join(', ', grep { defined } @mail_addresses );