It's looking for keys in the $abc hashref whose corresponding value is a hashref.

my $abc = {
    1 =>                 2,
    3 =>            [4, 5,],
    6 => {7 => 8, 9 => 10,},
};
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The condition in your code will be true for key 6, but false for the others, because their values are a plain scalar and an arrayref, correspondingly.