Incidentally the MP3 file format is designed so that it should be possible to concatenate files by simply writing one after the other; players must skip frames they can't process so they'd just ignore the intervening header. However, this only works if the encoding is exactly the same regarding sample rate, bps and $DEITY knows what else, so in practice it usually doesn't. Unless it's really necessary to do it in Perl, I'd second
in recommending some external audio editor such as SOX.