Thanks. I've added:

sub __args
{
    my( $x, $y, $rev )= @_;
    $y= $x->new( $y )
        if  ! ref $y;
    ( $x, $y )= ( $y, $x )
        if  $rev;
    return( $x, $y );
}
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So that boiler-plate reduces to: my( $x, $y )= __args( @_ ); in 6 places.