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    print "1st run\n" if exists $a{a};
    print "2nd run\n" if exists $a{a}{b}{c};
    print "3rd run\n" if exists $a{a};

# prints 3rd run

Note that the EXPR can be arbitrarily complicated as long as the
final operation is a hash or array key lookup or subroutine
...
This surprising autovivification in what does not at first--or
even second--glance appear to be an lvalue context may be fixed
in a future release.