$ perl -MO=Concise
my $s = 'foo';
my $string = "${s}bar";
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -:1) v ->3
5     <2> sassign vKS/2 ->6
3        <$> const(PV "foo") s ->4
4        <0> padsv[$s:1,3] sRM*/LVINTRO ->5
6     <;> nextstate(main 2 -:2) v ->7
b     <2> sassign vKS/2 ->c
-        <1> ex-stringify sK/1 ->a
-           <0> ex-pushmark s ->7
9           <2> concat[t3] sK/2 ->a
7              <0> padsv[$s:1,3] s ->8
8              <$> const(PV "bar") s ->9
a        <0> padsv[$string:2,3] sRM*/LVINTRO ->b
- syntax OK

$ perl -MO=Concise
my $s = 'foo';
my $string = $s . 'bar';
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -:1) v ->3
5     <2> sassign vKS/2 ->6
3        <$> const(PV "foo") s ->4
4        <0> padsv[$s:1,3] sRM*/LVINTRO ->5
6     <;> nextstate(main 2 -:2) v ->7
b     <2> sassign vKS/2 ->c
9        <2> concat[t3] sK/2 ->a
7           <0> padsv[$s:1,3] s ->8
8           <$> const(PV "bar") s ->9
a        <0> padsv[$string:2,3] sRM*/LVINTRO ->b
- syntax OK