I think you may have misread it: that's an equality test in the last line, not an assignment =)

It returns the value of the last statement in the sub, i.e, the value of

$num == int($num);
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Which is true (== 1) if $num and the integer part of $num are equal to each other, and false (==0)otherwise.

Philosophy can be made out of anything. Or less -- Jerry A. Fodor