C:\WINDOWS\system32>perl -MO=Concise -E"exists $h{a}"
4  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 74 -e:1) v:%,{,469764096 ->3
-     <1> ex-exists vK/1 ->4
3        <+> multideref($h{"a"}) vK/EXISTS ->4
-           <0> ex-gv s ->3
-e syntax OK

C:\WINDOWS\system32>perl -MO=Concise -E"$h{a}"
4  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 74 -e:1) v:%,{,469764096 ->3
-     <1> ex-helem vK/2 ->4
3        <+> multideref($h{"a"}) vK ->4
-           <0> ex-gv s ->3
-e syntax OK