nice list, but why not changing the argument alias in place to only return the error code?

  DB<101>  $var=2
 => 2

  DB<102> sub func {
               if ( $_[0] > 0 ) {
                  $_[0]--;
                  return OK_CODE;
               } 
               return ERR_CODE;
           } 

  DB<103> $err = func ($var);
 => "OK_CODE"

  DB<104> $err = func ($var);
 => "OK_CODE"

  DB<105> $err = func ($var);
 => "ERR_CODE"
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