$ perl -e " $a{0} ||= scalar keys %a; print $a{0} "
1
$ perl -e " $a{0} += scalar keys %a; print $a{0} "
1
$ perl -e " $a{0} = $a{0}+ scalar keys %a; print $a{0} "
0
$ perl -e " $a{0} = $a{0} || scalar keys %a; print $a{0} "
0


##</code><code>##

$ perl -MO=Concise -e " $a{0} ||= scalar keys %a; print $a{0} "
j  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
-     <1> null vK/1 ->c
7        <|> orassign(other->8) vK/1 ->c
6           <2> helem sKRM/2 ->7
4              <1> rv2hv sKR/1 ->5
3                 <#> gv[*a] s ->4
5              <$> const[PV "0"] s ->6
b           <1> sassign sK/BKWARD,1 ->c
-              <1> scalar sK/1 ->b
a                 <1> keys[t4] sK/1 ->b
9                    <1> rv2hv[t3] lKRM/1 ->a
8                       <#> gv[*a] s ->9
c     <;> nextstate(main 1 -e:1) v:{ ->d
i     <@> print vK ->j
d        <0> pushmark s ->e
h        <2> helem sK/2 ->i
f           <1> rv2hv sKR/1 ->g
e              <#> gv[*a] s ->f
g           <$> const[PV "0"] s ->h
-e syntax OK


##</code><code>##

$ perl -MO=Concise -e " $a{0} = $a{0} || scalar keys %a; print $a{0} "
n  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
f     <2> sassign vKS/2 ->g
-        <1> null sK/1 ->b
7           <|> or(other->8) sK/1 ->b
6              <2> helem sK/2 ->7
4                 <1> rv2hv sKR/1 ->5
3                    <#> gv[*a] s ->4
5                 <$> const[PV "0"] s ->6
-              <1> scalar sK/1 ->-
a                 <1> keys[t5] sK/1 ->b
9                    <1> rv2hv[t4] lKRM/1 ->a
8                       <#> gv[*a] s ->9
e        <2> helem sKRM*/2 ->f
c           <1> rv2hv sKR/1 ->d
b              <#> gv[*a] s ->c
d           <$> const[PV "0"] s ->e
g     <;> nextstate(main 1 -e:1) v:{ ->h
m     <@> print vK ->n
h        <0> pushmark s ->i
l        <2> helem sK/2 ->m
j           <1> rv2hv sKR/1 ->k
i              <#> gv[*a] s ->j
k           <$> const[PV "0"] s ->l
-e syntax OK