With this

 return $a and not $b
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The return happens first, and regardless of the value of $a. The rest of the line is therefore never even considered.

With this: $a and not $b the and is evaluated first, then the not $b if $a is false; then whatever is the result of that is returned by virtue of 'falling off the end' of the subroutine.

You might use return $a && !$b instead.