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

$ perl -MO=Concise -e " $a{0} ||= scalar keys %a; print $a{0} "
j  <@> leave[1 ref] vKP/REFC ->(end)
...
e              <#> gv[*a] s ->f
g           <$> const[PV "0"] s ->h
-e syntax OK

$ perl -MO=Concise -e " $a{0} = $a{0} || scalar keys %a; print $a{0} "
n  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
...
i              <#> gv[*a] s ->j
k           <$> const[PV "0"] s ->l
-e syntax OK