my $dir = 'Startdir';

list_dir($dir);

sub list_dir {
    my ($current) = @_;
    opendir my $dirh, $current or die $!;
    while( ... ) {    # wie bei Dir im Programm
        # . und .. ausschließen

        if ( -d "$current/$entry" ) {
            list_dir( "$current/$entry" );
        }
    }
    closedir $dirh;
}