lanx:~$ perl -MO=Concise -e 'my (0?$a:$b)="X";print $a' 
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <2> aassign[t3] vKS ->8
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[PV "X"] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
6           <0> padsv[$b:1,2] lPRM*/LVINTRO ->7
8     <;> nextstate(main 2 -e:1) v:{ ->9
b     <@> print vK ->c
9        <0> pushmark s ->a
a        <0> padsv[$a:1,2] l ->b
-e syntax OK

##</code><code>##

lanx:~$ perl -MO=Concise -e 'my ($b)="X";print $a' 
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <2> aassign[t2] vKS ->8
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[PV "X"] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
6           <0> padsv[$b:1,2] lPRM*/LVINTRO ->7
8     <;> nextstate(main 2 -e:1) v:{ ->9
b     <@> print vK ->c
9        <0> pushmark s ->a
-        <1> ex-rv2sv sK/1 ->b
a           <#> gvsv[*a] s ->b
-e syntax OK

##</code><code>##

perl -wce 'my $a if 0; print $a' 
Deprecated use of my() in false conditional at -e line 1.
-e syntax OK