...
sub groupsOf (&$@);
...
sub groupsOf (&$@)
{
    my $rcToRun  = shift;
    my $groupsOf = shift;

    my $rcDoIt;
    $rcDoIt = sub
       {
           $rcToRun->( map shift, 1 .. min scalar( @_ ), $groupsOf ),
           @_ ? &$rcDoIt : ();
       };
    &$rcDoIt;
}
...