π can't be used as a number because it already parses as an identifier (greek letter) and you can already use it as a sub name.

∞ does not, however, and simply emits a syntax error. You can use it as the delimiter for quote constructs, but that only applies after the beginning token of a quote construct. I'm very narrowly talking about changing perl's failure mode when it encounters this character to do something useful, since infinity is really a character that unambiguously indicates a mathematical value that also has an unambiguous floating-point encoding.

I'd be happy to see many more codepoints given language functionality, if they are unambiguous symbols that imply unambiguous scalar values. I can't name any others offhand. (Mathematicians really ought to stop stealing greek letters for things. I mean it's not like they were using mechanical tools with a limited range of symbols when they started this stuff... they could draw anythign they wanted. And, Greek is still an active language! not a dead one like Latin...)