$ perl -MO=Concise,-exec -we' $foo.=<STDIN>'
Name "main::foo" used only once: possible typo at -e line 1.
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v
3  <#> gvsv[*foo] s
4  <#> rcatline[*STDIN] sS
5  <@> leave[1 ref] vKP/REFC
-e syntax OK