I checked the source, and the $pdf->corefont() method pulls from 14 built-in fonts in the module itself...where their data is encoded in .pm files, here:
So, it's not looking at system installed fonts at all.Your issue is within the module itself.
Since your code is working for others, I would guess you've got a borked installation of the module. You could find try the ttffont() or psfont() method instead, assuming you have ttf or pfm files for the desired fonts around. The convert utility shows the location of these. Or perhaps try reinstalling the module?