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

##</code><code>##

./mytest.pl 123.2
123.2 is a number

./mytest.pl aBcd
aBcd isn't a number