Hmm?

use List::Util qw(max);
my $max_len_keys = max map length, keys %hash;
my $max_len_vals = max map length, values %hash;
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In cases like this where I specifically expect a "ref to X" I might even consider using a prototype (in this case, \%;$), then you can get rid of your own type checking and it will work regardless of any magic attached to whatever is passed in.

Makeshifts last the longest.