Create the largest integer number that the current Perl interpreter supports.

If you aren't picking a specific maximum number but just want to allow whatever the running perl does then you just need a function like this valid_int here:

use strict;
use warnings;

use Test::More tests => 5;

ok valid_int ('18446744073709551614'), '1 under max int';
ok valid_int ('18446744073709551615'), 'exactly max int';
ok ! valid_int ('18446744073709551616'), '1 over max int';
ok ! valid_int ('foo'), 'NaN';
ok ! valid_int ('1.3'), 'float';

sub valid_int {
    my $num = shift;
    return unless $num =~ /^\d+$/a;
    return int $num eq $num;
}
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The hard-coded values are just to test because I am on a 64-bit perl, adjust them to your own testing environment as appropriate.