If you were to use CGI.pm's param() method to retrieve form values and if you were to name all checkboxes the same, you could use a hash slice. Any day you can do that is a good day.

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 );
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