According to http://www.bribes.org/perl/ppmdir.html the PPM package was "compiled with SDL-1.2.9, SDL_image-1.2.4, SDL_gfx-2.0.13, SDL_mixer-1.2.6, SDL_net-1.2.5, SDL_ttf-2.0.7". There's no mention of smpeg there - so I'm wondering whether that particular build of SDL_Perl supports mp3 at all.
You could check with the guy who built the package. There's a contact email address at the bottom of the bribes web page.