As far as I can tell, the my attribute needs to be declared at compile time. Solo's example works because the trinary operator is indeed optimized away, and the parse tree looks like:

1  <0> enter 
2  <;> nextstate(main 1 -e:1) v
3  <0> padsv[$foo:1,2] vPM/LVINTRO
4  <@> leave[1 ref] vKP/REFC
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There is no "my" op. All "my" does is set some flags in the SV structure. The S_my_kid function in op.c doesn't seem to generate any ops - all it does it set the flags at compile time, and since that expression can't be evaluated at compile time Perl complains.