use Carp ();

sub WARN  () { 1 }
sub ERROR () { 2 }

sub DEBUG     () { 0     }  # True of false.
sub THRESHOLD () { ERROR }  # Or use $THRESHOLD. No real diff.

sub log_error {
   my $level = (@_==2 ? pop(@_) : THRESHOLD);
   return if $level < THRESHOLD;
   goto &Carp::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.