Then why does the string "65" print out "ok"? I didn't eval or convert it first.

Because 65 looks like a number but 0x65 does not. See perlnumber, http://perldoc.perl.org/perlapi.html#grok_number_flags (the meat of http://perldoc.perl.org/perlapi.html#looks_like_number, maybe even on truthiness

You can always change this with overload::constant, Numification of strings