There are lots of ways to do this. Here's one which matches how Perl works internally when it needs to decide if something is numeric:

   use Scalar::Util qw(looks_like_number);

   if (looks_like_number($foo)) {
       print "It could be a number.\n";
   } else {
       print "It's just a string.\n";
   }
[download]

Other solutions could involve regexes from Regexp::Common or coded by hand.

-sam