use warnings;
use strict;

my $x = shift;
length($x) && $x=~/\A[0-9]+\z/ or die "Need a non-negative integer";
my $y = "1000000000000000000000000000000000000000"; # just for demo

die "Integer too big" if length($x)>length($y)
	or sprintf("%0*s", length($y), $x) gt $y;
print "$x is ok\n";

# *now* you can treat $x like a number