# (a) could change entirely if run multiple times
( rand < .5 ? $a : $b ) = ( rand < .5 ? $a : $b );

# (a) might have side effects
bar() = bar() || foo();
sub bar :lvalue { ... }

# (a) might have side effects
tie $a, ...;
($a) = $a || foo();

##</code><code>##

perl -MO=Concise -e '($a) ||= foo()' | idealized-optree

leave
   enter
   nextstate
      orassign # ||=
         gvsv    # << $a
         sassign # scalar =
            entersub # foo()
                  pushmark
                     gv # *foo