$ perl -MO=Concise,-exec -we'my $foo = scalar <STDIN>'
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v
3  <#> gv[*STDIN] s
4  <1> readline[t3] sK/1
5  <0> padsv[$foo:1,2] sRM*/LVINTRO
6  <2> sassign vKS/2
7  <@> leave[1 ref] vKP/REFC
-e syntax OK
~$ perl -MO=Concise,-exec -we'my $foo = <STDIN>'
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v
3  <0> padsv[$foo:1,2] sRM*/LVINTRO
4  <#> gv[*STDIN] s
5  <1> readline[t3] sKS/1
6  <@> leave[1 ref] vKP/REFC
-e syntax OK

##</code><code>##

$ 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