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use Carp ();

sub log_error {
   goto(&Carp::carp) if pop(@_) >= THRESHOLD;
}

sub DEBUG     () { 0 }
sub FATAL     () { 4 }
...
sub THRESHOLD () { 0 }

log_error($msg) if FATAL >= THRESHOLD;

use Carp ();

...
log_error($msg) if DEBUG;  # Whole statement removed at compile time.
log_error($msg, WARN);     # Calls log_error, but not carp.
log_error($msg, FATAL);    # Calls log_error and carp.