Modifying Foo::Bar to this:

package Foo::Bar;

use Moose;
extends 'Foo';

use Data::Dumper;

around 'BUILDARGS' => sub {
    my ($orig, $class, %args) = @_;
    print STDERR "Foo::Bar::BUILDARGS running.\n";

    $args{x} = 25;

    return $class->$orig(%args);
};

1;
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Seems to do what I want. Foo::BUILD is only executed once and it receives the updated 'x' value prior to BUILD execution, so 'y' is only calculated once. Here is the output in this case:

plxc16479> ex.pl
Foo::Bar::BUILDARGS running.
Foo::BUILD running
$VAR1 = bless( {
                 'y' => '5',
                 'x' => 25
               }, 'Foo::Bar' );
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