I agree with the notion that calling a logging function which immediately returns shouldn't generate a much overhead. In a tight loop with lots of iterations (something that could make the overhead significant), i usually don't log on every iteration, only every 1000 or so:

for(my $i = 0; $i < 1_000_000; $i++) {
    if($i % 1000 == 0) {
        debug("Foo $i");
    }
    # do some stuff
}
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