Hi manbroski,

"..Notice that I have to dereference it after the passage.." Why is that?

Did you also notice your subroutine sort_and_print_hash_keys definition

sub sort_and_print_hash_keys (\%) {..
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and how you eventually used the it like so:

sort_and_print_hash_keys(%letters); # you passed a hash variable not a
+ hash ref.
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".. But as far as the prototype passing, I was certain that it enforces a pass by reference..."

If I may suggest, you will do well yielding the wisdom of jwkrahn, as regard the usage of prototype for this reason:

When you use a reference prototype, like "\$", "\@", "\%" "...those symbols don't actually say that you must pass in a scalar reference, an array reference, and a hash reference. Rather, they say you must pass in a scalar variable, an array variable, and a hash variable. That means that the compiler insists upon seeing a properly notated variable of the given type, complete with "$", "@", or "%" in that slot. You must not use a backslash. The compiler silently supplies the backslash for you... "
from Far More than Everything You've Ever Wanted to Know about Prototypes in Perl -- by Tom Christiansen
-- by liverpole, under subheading Problems with Reference Prototypes

Hope this helps.