> Scalar::Util's looks_like_number method will do what you want.

Scalar::Util uses a regex to get the answer. I'm not sure why snoopy's request has that requirement, but...

ben@Tyr:~$ perl -wne'print if /sub looks_like_number/../}/' `perldoc -
+l Scalar::Util`
sub looks_like_number {
  local $_ = shift;

  # checks from perlfaq4
  return 0 if !defined($_) or ref($_);
  return 1 if (/^[+-]?\d+$/); # is a +/- integer
  return 1 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/); # 
+a C float
  return 1 if ($] >= 5.008 and /^(Inf(inity)?|NaN)$/i) or ($] >= 5.006
+001 and /^Inf$/i);

  0;
}
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