The algorithm I would use to calculate BPM is to
- calculate the modulation envelope of the waveform using either a Hilbert transform or rectification and smoothing
- compute the Fast Fourier Transform (FFT) of the modulation envelope
- look for the strongest peak in the 0.2-5Hz range (depending on the music)
- Assign the center of the peak to be the BPM.
Note that most music is approximately scale-free (or fractal, to use a more popular term) so that there will be energy at all time scales. But for some kinds of music there will be a prominent peak in the above range that people usually associate with BPM.
How to do all of this in perl? I would use PDL. It is very efficient in manipulating vector quantities like sound waveforms and has a built in FFT function.
