$ perl -MO=Concise,-exec -Mv5.40 -Mbuiltin=inf -e'my $x = inf;'
Built-in function 'builtin::inf' is experimental at -e line 1.
1  <0> enter v
2  <;> nextstate(main 9 -e:1) v:us,*,&,{,$,fea=8
3  <$> const[NV Inf] s
4  <1> padsv_store[$x:9,10] vKS/LVINTRO
5  <@> leave[1 ref] vKP/REFC
-e syntax OK