G'day Dean,

"List::Util::pairmap would be an existing solution very similar to the new feature. It also seems to run quite quickly (but I can't compare it to for_list since I don't have a new-enough perl)."

No problem. I have v5.36.0 installed. I added your code to the OP benchmark code.

Mine starts as follows. Refer to "perl5360delta: use v5.36" for why I removed the strict and warnings pragmata.

#!/usr/bin/env perl

use v5.36;

use List::Util qw /pairs pairvalues pairmap/;
[download]

I then added:

• say a_pairmap(); after the existing eight say statements.
• a_pairmap     => \&a_pairmap, to the end of the cmpthese hashref.
• Your sub a_pairmap { ... } verbatim, at the end of the script.

I ran the benchmark five times. All results were pretty much the same; here's a median set:

2500050000
2500050000
2500050000
2500050000
2500050000
2500050000
2500050000
2500050000
2500050000
                Rate a_pairs_alias a_pairs h_each h_by_key h_for_list 
+h_vals a_pair_vals a_pairmap a_for_list
a_pairs_alias 48.8/s            --     -4%   -67%     -73%       -83% 
+  -90%        -90%      -92%       -94%
a_pairs       51.0/s            4%      --   -66%     -71%       -82% 
+  -90%        -90%      -91%       -94%
h_each         150/s          207%    193%     --     -16%       -47% 
+  -69%        -71%      -74%       -82%
h_by_key       178/s          265%    249%    19%       --       -37% 
+  -64%        -65%      -69%       -78%
h_for_list     284/s          482%    457%    90%      59%         -- 
+  -42%        -45%      -51%       -66%
h_vals         490/s          903%    860%   227%     175%        72% 
+    --         -5%      -16%       -41%
a_pair_vals    513/s          951%    906%   243%     188%        81% 
+    5%          --      -12%       -38%
a_pairmap      582/s         1091%   1041%   289%     226%       105% 
+   19%         13%        --       -30%
a_for_list     827/s         1594%   1521%   453%     364%       191% 
+   69%         61%       42%         --
[download]

In all runs, a_pairmap was second fastest; and, h_by_key was faster than h_each (which differs from the OP results).

My environment:

$ perl -v | head -2 | tail -1
This is perl 5, version 36, subversion 0 (v5.36.0) built for cygwin-th
+read-multi

$ uname -a
CYGWIN_NT-10.0 titan 3.2.0(0.340/5/3) 2021-03-29 08:42 x86_64 Cygwin
[download]