A very simple method for checking whether a number $N is odd or even is to ask yourself "is there any remainder when I divide $N by 2?".

So, for example:

if (0 == $N % 2) {
    print "The number $N is even\n";    # There is no remainder
} else {
    print "The number $N is odd\n";     # There is a remainder (of 1)
}
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