$ 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