Scalar::Util has a function looks_like_number() for just this purpose. It is included in Perl 5.8, or you can get it from CPAN.

You can use looks_like_number() in conjunction with int() to tell what type of number you have.

#!/usr/bin/perl

use strict;
use warnings;
use Scalar::Util qw(looks_like_number);

my $x = $ARGV[0];

if (looks_like_number($x)) {
    if (int($x) == $x) {
        print("$x is an integer\n");
    } else {
        print("$x is numeric\n");
    }
} else {
    print("Your input was not numeric\n");
}
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