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
ww in recommending some external audio editor such as SOX.