I came up with the following:

use warnings;
use strict;

while (<DATA>) {

   s|^(?:<bl>)?(.*?)(?:</bl>)?$|<listitem>$1</listitem>|;
   print
}
__DATA__
<bl>List item 1
List item 2
<bl>List item a
List item b
List item c</bl>
List item 3</bl>
[download]

It does not produce exactly the intended output, but:

<listitem>List item 1</listitem>
<listitem>List item 2</listitem>
<listitem>List item a</listitem>
<listitem>List item b</listitem>
<listitem>List item c</listitem>
<listitem>List item 3</listitem>
[download]

Which, to me at least, looks more likely to be useful.