Including 'expensive_function' in a benchmark means that you are mostly measuring how often that function gets called (which isn't what you asked for) and reduces the differences in the remaining items to the level of noise (which a worse benchmark).

I commented out the guts of expensive_function, shortened the names, added tye2 which does what I thought was the obvious optimization of pulling out the first char of $str_a at the top: 

    'tye2' => sub {
        my $f = ord($str_a);
        for my $str_b ( @list ) {
            next if $f != ord( $str_b );
            expensive_function ( $str_a, $str_b );
        }
    },

[download]
and got these results: 
        Rate   hb1   hb2  L~R3   BUk  L~R2  dlrm   L~R  tye1 kvale  ty
+e2
hb1   46.5/s    --  -35%  -35%  -38%  -60%  -62%  -70%  -73%  -73%  -7
+4%
hb2   71.4/s   53%    --   -0%   -6%  -38%  -42%  -54%  -59%  -59%  -6
+1%
L~R3  71.4/s   53%    0%    --   -6%  -38%  -42%  -54%  -59%  -59%  -6
+1%
BUk   75.7/s   63%    6%    6%    --  -35%  -39%  -51%  -56%  -56%  -5
+8%
L~R2   116/s  149%   62%   62%   53%    --   -6%  -25%  -33%  -33%  -3
+6%
dlrm   124/s  166%   73%   73%   63%    7%    --  -20%  -28%  -29%  -3
+2%
L~R    155/s  233%  117%  117%  105%   34%   25%    --  -10%  -11%  -1
+4%
tye1   172/s  270%  141%  141%  128%   48%   39%   11%    --   -1%   -
+5%
kvale  174/s  274%  143%  143%  130%   50%   41%   12%    1%    --   -
+4%
tye2   181/s  289%  154%  154%  139%   56%   46%   17%    5%    4%    
+--

[download]

But I still consider such nano-optimization games to be more a waste than of value. (:

- tye        

In reply to Re^2: Matching First Character of Strings Efficiently (benchmarking) by tye
in thread Matching First Character of Strings Efficiently by Limbic~Region

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