$ perl -MO=Deparse -e '(-s "foo")+100' 
-s('foo') + 100;
-e syntax OK

$ perl -w -MO=Concise -e '(-s "foo")+100' 
Useless use of addition (+) in void context at -e line 1.
7  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
6     <2> add[t2] vK/2 ->7
4        <1> ftsize[t1] sKP/1 ->5
3           <$> const[PV "foo"] s ->4
5        <$> const[IV 100] s ->6
-e syntax OK

$ perl -w -MO=Concise -e '-s ("foo")+100' 
7  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
6     <1> ftsize[t2] vK/1 ->7
5        <2> add[t1] sK/2 ->6
3           <$> const[PVNV "foo"] sP ->4
4           <$> const[IV 100] s ->5
-e syntax OK