As already noted, you needed to read the directory contents again. The use of the <*.mp3> globbing operator can cause some unwanted side-effects, at least in my experience (maybe on older versions of Perl?). The problem is that the shell is used to do the filename expansion, and if there are lots of files that match, the list can be truncated. I usually use something like:

opendir(INDIR, "$INDIR");
@mp3files = grep(/\.mp3$/, readdir(INDIR));
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to avoid this potential pitfall. If you only have a small number of files, shouldn't be a problem.