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# warning: untested
my %collection;
...
   my ($artist, $album, $song) = split;
   push @{$collection{$artist}{$album}}, $song;
}

foreach my $artist (keys %collection) {
   print "$artist:\n";
...
      print join("\n\t\t", @{$collection{$artist}{$album}}), "\n";
   }
}