You can avoid that behaviour by using a capturing pattern. 
$ perl -le'print join " : ", split /(\|)/, "2|3|||||"'
2 : | : 3 : | :  : | :  : | :  : | :  : |

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This is also documented in perldoc -f split. Of course now every other element is the separator, which isn't what we wanted. So out with them: 
$ perl -le'print join " : ", grep ++$i%2, split /(\|)/, "2|3|||||"'
2 : 3 : : : :

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There you go.

Update: Yikes! I caught a mistake: the last field still disappears if empty. Count the pipes in the string and the colons in the output.. At this point we lose some grace.. The oneliner is ugly: 

$ perl -le'print(join(" : ", grep(++$i%2, split /(\|)/, "2|3|||||"), (
+"")x(1-$i%2)))'
2 : 3 : : : : :

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while the multiliner is awkward: 
$ perl -le'my @field = grep ++$i%2, split /(\|)/, "2|3|||||"; push @fi
+eld, "" unless $i%2; print @field'
2 : 3 : : : : :

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Oh well.

Makeshifts last the longest.

In reply to Re^3: split/map weirdness: empty strings vs undef by Aristotle
in thread split/map weirdness: empty strings vs undef by hossman

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