Better yet, use a constant instead of a hashref lookup, and the whole if block will be obliterated at compile time when you disable debugging.

use constant DEBUG => 0;
...
if (DEBUG) {
    # complex logging, which disappears from the op tree
}
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FWIW, my experience has been like Solo's, where you end up inadvertently evaluating the arguments to the logging methods, and this takes a lot of time (or a little bit of time in a tight loop that's called a zillion times).