$${a} and $$a compile to the same stuff. The first expressions contains more tokens so it takes longer to parse but you parse it only once (admittedly per script execution) But the real point is that is also more difficult to read.

# perl -MO=Concise,-exec -e '$$a'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v
3  <$> gvsv(*a) s
4  <1> rv2sv vK/1
5  <@> leave[t1] vKP/REFC
-e syntax OK
 perl -MO=Concise,-exec -e '${$a}'
1  <0> enter
2  <;> nextstate(main 2 -e:1) v
3  <$> gvsv(*a) s
4  <1> rv2sv vK/1
5  <@> leave[t1] vKP/REFC
-e syntax OK
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-- stefp