Adding those additional restrictions has no effect. This is my current $schema:

my $schema = {
    section => { 
        type => 'nested',
        child => { 
            key => { 
                type  => 'string',
                regex => qr/^ ARRAY \( 0x\p{HexDigit}{1,8} \) $/x,
            },   
        },   
    },   
};
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The code still runs to completion. If I make the regex different (e.g. by changing "ARRAY" into "ARAY"), the code complains about the data not matching the schema. Looks like a bug to me...

Thank you for giving me the clue!