If you're going to use a regexp to parse a number (which seems to me to be the right approach as well), the best bet would be to use Conway's Regexp::Common module (actually Regexp::Common::number but that link dumps you into a page of results) which does this for you (and one would assume that it gets all the hairy bits right).

   use Regexp::Common qw/number/;

   sub make_num {
      return $_[0] =~ /(^$RE{num}{real}$)/ ? $1 : undef;
    }
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tweaked: s/is_num/make_num/ and changed the return value, in order to be more in line with the original question.