> perl -MO=Deparse,-q
use warnings;
my (%a, %b, $c);
my $x = "$a{'foo'} $b{'bar'} $c"
__END__
use warnings;
my(%a, %b, $c);
my $x = $a{'foo'} . ' ' . $b{'bar'} . ' ' . $c;
- syntax OK

##</code><code>##

lanx@nc10-ubuntu:~$ perl -w -MO=Concise
my (%a,%b,$c);
my $x = "$a{'foo'} $b{'bar'} $c";
o  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -:1) v:{ ->3
7     <@> list vKPM/128 ->8
3        <0> pushmark vM/128 ->4
4        <0> padhv[%a:1,3] vM/LVINTRO ->5
5        <0> padhv[%b:1,3] vM/LVINTRO ->6
6        <0> padsv[$c:1,3] vM/LVINTRO ->7
8     <;> nextstate(main 2 -:2) v:{ ->9
n     <2> sassign vKS/2 ->o
-        <1> ex-stringify sK/1 ->m
-           <0> ex-pushmark s ->9
l           <2> concat[t8] sKS/2 ->m
j              <2> concat[t7] sKS/2 ->k
h                 <2> concat[t6] sKS/2 ->i
d                    <2> concat[t5] sK/2 ->e
b                       <2> helem sK/2 ->c
9                          <0> padhv[%a:1,3] sR ->a           <-------
a                          <$> const[PV "foo"] s ->b
c                       <$> const[PV " "] s ->d
g                    <2> helem sK/2 ->h
e                       <0> padhv[%b:1,3] sR ->f              <-------
f                       <$> const[PV "bar"] s ->g
i                 <$> const[PV " "] s ->j
k              <0> padsv[$c:1,3] s ->l                        <-------
m        <0> padsv[$x:2,3] sRM*/LVINTRO ->n
- syntax OK

##</code><code>##

>  perl -w 
my (%a, %b, $c);  
$a{foo}="FIXED";
my $x = "$a{'foo'} $b{'bar'} $c";
__END__
Use of uninitialized value in concatenation (.) or string at - line 3.
Use of uninitialized value $c in concatenation (.) or string at - line 3.