Thank you - this is excellent. You have confirmed my suspicion that my tests were not good. Now I need to find a test that tricks the optimizer. Given that the ex-stringify sK/1 ->a op was removed, I guess using constant strings is not the way to test.

I always understood that 'abc' was better than "abc", because of the interpolation, but it seems that the optimizer sees through this, given the constant strings, so all I am doing is being kinder to the optimizer. Premature optimization.

The following now uses a string read from <>, and therefore not a constant:

% perl -MO=Concise
my $s=<>;
my $t="$s";
b  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -:1) v ->3
-     <1> null vKS/2 ->6
3        <0> padsv[$s:1,3] sRM*/LVINTRO ->4
5        <1> readline[t3] sKS/1 ->6
4           <#> gv[*ARGV] s ->5
6     <;> nextstate(main 2 -:2) v ->7
a     <2> sassign vKS/2 ->b
8        <@> stringify[t5] sK/1 ->9
-           <0> ex-pushmark s ->7
7           <0> padsv[$s:1,3] s ->8
9        <0> padsv[$t:2,3] sRM*/LVINTRO ->a
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I'll recreate tests based on this. Thank you all.