sub setRecursiveCopyCommand {
    my $OS = $^O; 
    
    if ( $OS eq 'VMS' ) {
       return whatever the hell they do in VMS 
    } elsif ( $OS eq 'PLAN9' ) {
       return whatever the hell they do in PLAN9
    } elsif ( $OS eq monkeys_fly_out_of_my_ass ) {
       you get the picture
    } else {
    ...
}