#!/usr/bin/perl
use strict;
use warnings;
use Errno;

sub usenumbers {
    my $var = shift;
    $var =~ /^\d+$/ or ($! = &Errno::EINVAL) and return;
    print "Yes, [$var] has numbers.\n";
    return 1;
}

for (qw(1 4 haha)) {
    usenumbers($_) or die "$_: $!";
}