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

Your assumptions about what happens why are incorrect.

$ perl -MO=Concise -e'my $s=""; my $t="$s";'
b  <@> leave&#91;$s:1,3] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
5     <2> sassign vKS/2 ->6
3        <$> const(PV "") s ->4
4        <0> padsv[$s:1,3] sRM*/LVINTRO ->5
6     <;> nextstate(main 2 -e:1) v ->7
a     <2> sassign vKS/2 ->b
8        <@> stringify[t3] sK/1 ->9
-           <0> ex-pushmark s ->7
7           <0> padsv[$s:1,3] s ->8
9        <0> padsv[$t:2,3] sRM*/LVINTRO ->a
-e syntax OK
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The "stringify" op is still active.