More precisely, it's one of the opcodes that implements $a[$i]. (There appears to be a aelemfast as well.)

$ perl -MO=Concise -e'$a[$i]'
7  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
6     <2> aelem vK/2 ->7
4        <1> rv2av sKR/1 ->5
3           <#> gv[*a] s ->4
-        <1> ex-rv2sv sK/1 ->6
5           <#> gvsv[*i] s ->6
-e syntax OK
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But oddly, there's no array indexing at the source line number given by the message.