...fwiw I would do this

use v6; 

class Point {
    has $.x is rw = 0;
    has $.y is rw = 0;

    method gist { "[$.x, $.y]" }

    method set(:$x, :$y) { 
        $.x = $x with $x; 
        $.y = $y with $y; 
    }   
}

class Pixel is Point {
    has Int $.x is rw = 0;
    has Int $.y is rw = 0;
    has $.color = '#000000';
}

my $point = Pixel.new();
say $point.x;  # 0 
say $point;    # [0, 0]
$point.x = 3;  # Works now!

$point = Pixel.new( :x(3), :y(5) );
say $point;  # [3,5]

$point.set( :x(10), :y(20) );
say $point;  # [10, 20] 

$point.set( :x(100) );
say $point;  # [100, 20]
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