$ perl -MO=Concise -e 'if ($x != -1 ){ print "foo" }'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 3 -e:1) v ->3
-     <1> null vK/1 ->a
6        <|> and(other->7) vK/1 ->a
5           <2> ne sK/2 ->6
-              <1> ex-rv2sv sK/1 ->4
3                 <#> gvsv[*x] s ->4
4              <$> const[IV -1] s ->5
-           <@> scope vK ->-
-              <0> ex-nextstate v ->7
9              <@> print vK ->a
7                 <0> pushmark s ->8
8                 <$> const[PV "foo"] s ->9
-e syntax OK
$ perl -MO=Concise -e 'if ($x == 0 ){ print "foo" }'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 3 -e:1) v ->3
-     <1> null vK/1 ->a
6        <|> and(other->7) vK/1 ->a
5           <2> eq sK/2 ->6
-              <1> ex-rv2sv sK/1 ->4
3                 <#> gvsv[*x] s ->4
4              <$> const[IV 0] s ->5
-           <@> scope vK ->-
-              <0> ex-nextstate v ->7
9              <@> print vK ->a
7                 <0> pushmark s ->8
8                 <$> const[PV "foo"] s ->9
-e syntax OK