• Complication: The closing LI tag is optional.
• Simplification: An LI element can only be closed by a few tags, they will always close an LI element, and one of them must be present. They are: </LI>, <LI>, </OL> and </UL>.
• Simplification: OL, UL and LI elements cannot be placed inside of a SPAN element.
• Complication: OL, UL and (indirectly) LI elements can be placed inside of an LI element.

our $ul_or_ol;
local $ul_or_ol = qr{
   (?:
      <ul \b
      (?:
         (??{ $ul_or_ol })
      |
         (?! </ul \b ).
      )*
      </ul \b [^>]* >
   |
      <ol \b
      (?:
         (??{ $ul_or_ol })
      |
         (?! </ol \b ).
      )*
      </ol \b [^>]* >
   )
}xi;

my $re = qr{
   <li \b [^>]* >

   <span class="[^"]+">
   [^<]+
   </span>

   (
      (?:
         $ul_or_ol
      |
         (?! < (?:li|/li|/ol|/ul) \b ).
      )*
   )

   (?:
      </li \b [^>]* >
   |
      (?= < (?:li|/ol|/ul) \b )
   )
}xi;
[download]

• Bug/Assumption: The above can fail if there's a < or a > inside an attribute value, a comment, a script or a style.
• Bug/Assumption: The above can fail if some valid SGML constructs are used. Fortunately, noone uses them.
• Bug: Unmaintainable.
• Note: It can be simplified if more assumptions are made. It can be optimized as well.

Untested.

Update: Fixed to handle nested lists in the "MATCH HERE" portion.