For your original question, comparing with the abs value should do the trick ...

if ( $num == abs($num) ) {
    print "positive";
} else {
    print "negative";
}
[download]

... but for your revised question, you should check the modulus ...

if ( $num % 2 ) {
    print "odd";
} else {
    print "even";
}
[download]

However, if all you really want to do is alternate colours...