$foo = def( $bar, $default_for_foo );

##</code><code>##

    $a = has( a=>$hashref, $default_for_a );

##</code><code>##

sub has {
    my( $key, $hv, $val, $force )= @_;
    if(  exists $hv->{$key}  ) {
        $val= $hv->{$key};
    } elsif(  ! defined wantarray  ||  $force  ) {
        $hv->{$key}= $val;
    }
    return $val;
}