I have no idea what it's supposed to do. I couldn't find anything in my local docs about it either (nor about NaN, which has similar behavior on my build).

Maybe it depends on the local floating-point arithmetic library?

Searching came up with: this perl5.porters msg, and also FMTYEWTK about is_numeric, which seems to suggest that perl calls to the atof() library function. Finally, bignum suggests that "Perl doesn't always handle bareword NaN/inf properly."