DB<7> $curdir = 'Y:\Music\Schubert\Lieder\Terfel';

  DB<7> $startdir = 'Y:\Music';

  DB<8> $out = join "_", map {s/^\Q$startdir\E\\//i;  split /\\/, "$_.pls"} $curdir ;

  DB<9> p $out
Schubert_Lieder_Terfel.pls