$a = 123;
print $a =~ m/^\d+$/, "\n";

$a = "abc";
$ret = ($a =~ m/^\d+$/);
if (defined($ret)) {
    print "=$ret=\n";#print out ==
} else {
    print "undef";#not going this loop
}

##</code><code>##

sub true_false {
    if (shift()) {
        print "true\n";
    } else {
        print "false\n";
    }
}

true_false("a");
true_false(1);
true_false("");
true_false(0);

if (undef) { #I don't want to use true_false for this case, so it is guranteed that shift() does not play any magic
    print "true\n";
} else {
    print "false\n";
}