Interesting. I have this thing about not doing 0 == blah because I think that reads better as !blah so I wouldn't have had the problem. In many C++ish languages the two are essentially the same so it's pretty much personal preference between an ugly verbose numeric compare and a clean succinct boolean test. In Perl, as you may have found, it can be more subtle than that!

That said, how were you performing your empty test? The simple version works fine for me:

use strict;
use warnings;

my %hash;

print "$^V\n";
print "Empty: !\%hash\n" if !%hash;
print "Empty: ! scalar\n" if ! scalar %hash;
print "Empty: 0 == scalar\n" if 0 == scalar %hash;
print "Empty: ! keys\n" if ! keys %hash;

++$hash{entry};
print "Not empty: \%hash\n" if %hash;
print "Not empty: scalar\n" if scalar %hash;
print "Not empty: 0 != scalar\n" if 0 != scalar %hash;
print "Not empty: keys\n" if keys %hash;
[download]

Prints:

v5.32.1
Empty: !%hash
Empty: ! scalar
Empty: 0 == scalar
Empty: ! keys
Not empty: %hash
Not empty: scalar
Not empty: 0 != scalar
Not empty: keys
[download]

Update: fixed mismatch between test and print for 0 == scalar