Tl;dr but

%myHash = &makeHash($key, $val, \%myHash)

Skip the left part and you are fine, no need to reset the hash. :)

 makeHash($key, $val, \%myHash)

update

Your makeHash() is seriously broken.

Please have a look at perlref to understand how references work.

update

Better (among many things)

my $subHashRef = shift @_; 
$subHashRef->{$key} = $val;
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