Tweakism to get something like this:

sub is_ok
{
        my ($x, $y) = @_;

        my %x;

        @x{@$x} = 1;

        return !grep { exists($x{$_}) } @$y;
}
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You seem to create a hash and then just iterate over its keys, instead of just using the straight list. Here I just make one hash and use the other list raw.

grep is used to produce a list of matches, which is then negated, so that it returns a true value if there are no matches.