Looping through all the keys in the hash is very inefficient. To check if $name is a key of %$all_names, you could use :

exists $$all_names{$name};
%$all_names ~~ $name; # Shortcut using the smartmatch operator
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Note that this isn't the same as defined $$all_names{$names}. If you do $$all_names{$key} = undef, exists would return true (because the key exists), but defined would return false (because the value is undefined).

To select elements from an array based on a condition, you can use grep { condition } @array. Putting it all together, to get all elements of @arr_of_names that are keys of %$all_names, you can try this :

@good = grep { %$all_names ~~ $_ } @arr_of_names;
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Note that inside the grep condition, the element being tested is aliased to $_.