You use regexes to check whether something is a valid number. The advantage of this is that you can easily adapt the regexes to whatever your interpretation of a number is. Here's an example out of the Cookbook:
print "$number is a C float!\n" if $number=~/^([+-]?)(?=\d|\.\d)\d*(\.
Chapter 2 of the Perl Cookbook
provides numerous recipes for dealing with numbers in Perl. It comes highly recommended.
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Want more info? How to link or
or How to display code and escape characters
are good places to start.