In addition to japhy's reference, you may find these helpful:

• The relevant section in perlfaq4. (Try perldoc perlfaq4 to read your local copy.)
• Recipe 2.1 in The Perl Cookbook (aka the Ram book)
• How to identify a number datatype in a string?
• Number?
• Validating Numbers in CGI Script?
• Checking whether a $var is a number
• Tom Christiansen's thoughts on the subject.
  (tilly and MeowChow refer to this article in the earlier threads, but the link they used is now out-of-date. This is its current location.)

--f