An empty list, in list context, is just that -- a list with 0 elements. In scalar context, though, something must be assigned, and undef is that assigned value. Indexing beyond the end of a list, in list context, returns an empty list, and undef in scalar context.

It is a possible oddity, though, that indexing an element of a list which is undef and assigning it to a list returns an empty list, and not the undef that was there -- this is probably because it was within the same blanket solution to the problem above. However, in 5.6.0, this buggish behavior was removed.

# 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 ()
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My golf approach would be:

# 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}}
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