Try this (a little condensed)...

if($in{'chkboxname1'}){
$in{'form-to'} .= ',group1@domain.com';
}
if($in{'chkboxname2'}){
$in{'form-to'} .= ',group2@domain.com';
}
if($in{'chkboxname3'}){
$in{'form-to'} .= ',group3@domain.com';
$in{'form-to'} =~ s/^,//;

--lnl