How do I determine whether a scalar is a number/whole/integer/float?

Assuming that you don't care about IEEE notations like ``NaN'' or ``Infinity'', you probably just want to use a regular expression. 

   warn "has nondigits"        if     /\D/;
   warn "not a whole number"   unless /^\d+$/;
   warn "not an integer"       unless /^[+-]?\d+$/
   warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
   warn "not a C float"
       unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

Or you could check out CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz instead.