# 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}}