Perl will usually 'do the right thing' if you use '/' as the directory separator. This way I can use your script on my linux box and Mac OS X laptop as well as my W2K system without edits:

chdir "d:/music/new";
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chdir "d:\\music\\new";
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Instead of hard coding the directory (or using Getopt::*), you could do:

my $music_dir = shift || "d:/music/new";
chdir $music_dir;
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Which allows 'mp3_sorter.pl' (to process d:\music\new) as well as 'mp3_sorter.pl /Volumes/myafp/mp3' (to process the mp3s on an AppleTalk share).

For little hacks, I'll embed string constants. But I try to make it easy to override them if at all possible.