Wouldn't it be easier to use a PD or shareware audio editor?

mp3 files have a header -- so to make one track of multiple tunes, you'll need (I think) to edit the header information so that your player knows it should continue past the end of the first tune (and either move the second tune's minimum-required header info to before that first tune or re-write your player to accept the need to skip over the second header... and so on and so forth... ad nauseum...
and that doesn't even begin to deal with footers (if any).

CPAN does, however, hold some promise, despite the fact that most items with mp3 in the name seem to deal with tagging, not editing.

• Audio::Digest::MP3
• (possibility?): MP3::Album::Track
• Audio::MPEG
• etc, maybe.

If you're not familiar with CPAN, take a gander; you'll find it invaluable.