You understand perlguts far better than I, but there is a measurable penalty for interacting with a hash with locked keys:

#!/usr/bin/perl
use strict;
use warnings;

use 5.10.0;

use Hash::Util 'lock_keys';
use Benchmark 'cmpthese';

lock_keys my %hash1, qw/foo bar baz/;
my %hash2;

cmpthese(1e2, {
    'lock'      => sub { for (1 .. 1e5) {$hash1{foo}++;$hash1{bar}++;$
+hash1{baz}++;} },
    'no_lock'   => sub { for (1 .. 1e5) {$hash2{foo}++;$hash2{bar}++;$
+hash2{baz}++;} },
});
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          Rate    lock no_lock
lock    61.6/s      --     -8%
no_lock 66.8/s      8%      --
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... measurable penalty ...

Benchmark is tricky. When I looked at the output from your test, I thought "Heck, an 8% difference may be measurable, but it's sure not meaningful, not from Benchmark!"

I ran your code several times and got a range of advantages for  'no_lock' of 10% - 79%. Discarding outliers and doing an eyeball average gave me an advantage of about 35%. (I won't reveal the actual values I got because I don't think they're really significant; a much larger sampling of benchmark comparisons would be needed to achieve some sort of statistical significance.)

I then ran your code a few times with the  for (1 .. 1e5) { ... } loop limit set to 1e6. This runs several tens of seconds on my laptop. This produced a  'no_lock' advantage range of 4% - 25%, and an eyeball average sans outliers of about 15%.

Multiple runs of Benchmark produce a set of measurements I think is meaningful, and a massaged advantage of roughly 15% to 35% is what I would consider significant. I wouldn't dispute this point. The (rather long-winded) point I want to make is that Benchmark comparisons should always be made repeatedly in order to be meaningful, and should always be taken with a grain of salt.

---

Give a man a fish:  <%-{-{-{-<

Yes, it isn't trapped at compile time:

$ perl '-mfeature=say' -cw 1174218.pl
Name "main::var" used only once: possible typo at 1174218.pl line 7.
1174218.pl syntax OK
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