# in 5.005_02
$x = (1,2,3)[10];  # $x is undef
@x = (1,2,3)[10];  # @x is (), not (undef)
@x = (undef)[0];   # @x is (), not (undef)

# in 5.6.0
$x = (1,2,3)[10];  # $x is undef
@x = (1,2,3)[10];  # @x is (), not (undef)
@x = (undef)[0];   # @x is (undef), not ()

##</code><code>##

# returns the collapsed list: 15 chars
#             123456789_12345
sub collapse {grep{defined}@_}

# returns the collapsed list, via array ref: 20 chars
#             123456789_123456789_
sub collapse {grep{defined}@{+pop}}

# modifies array ref elements in place: 26 chars
#             123456789_123456789_123456
sub collapse {@$_=grep{defined}@{$_=pop}}