A function to determine if a string is numeric

juliosergio has asked for the wisdom of the Perl Monks concerning the following question:

Is there in Perl any function to determine if the contents of a string is numeric?
Something that behaves like:

#! /usr/bin/perl
# mytest.pl
$s = shift;
if (isnumber $s) {
  print "$s is a number\n";
} else {
  print "$s isn't a number\n";
}
[download]

Then when you call it:

./mytest.pl 123.2
123.2 is a number

./mytest.pl aBcd
aBcd isn't a number
[download]