Type::Tie does similar to your example tie code. And there's XS implementations for a lot of types in Type::Tiny::XS. I really should look at the possibility of implementing the tie stuff in XS too.

use Type::Tie;
use Types::Common::Numeric qw(PositiveOrZeroInt);

tie my $x, PositiveOrZeroInt;
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Combining it with Attribute::Handlers could be a fun idea.

Also, you probably want to use [0-9] in your regular expression, not \d. Yes, there's a difference.