As you do not give an example of the original text and of the result you would expect for it, it is very difficult to answer you.

A few ideas though:

• if your input is plain text, not XML, use either Parse::RecDescent or just plain regexps, coupled with XML::Writer or a SAX writer (XML::Handler::YAWriter or XML::SAX::Writer),
• if your input is already XML, then you can use a SAX filter to isolate the bits you want to process further, and treat them as plain text, printing the result or emitting SAX events on it,
• if you know a bit about SGML, you could try writing the SGML DTD and taking advantage of the minimization features (wherer the parser infers the markup from the DTD, using the DTD structure and for example line returns as an element delimiter) to see if by any chance your text is not already a valid SGML document, or cannot simply be made into. Going from SGML to XML is as simple as using sx (also called osx in some linux distributions),

And finally, because no post of mine is complete without a shameless XML::Twig plug, if all you want is wrap the lines in a list in the appropriate tags, then you can use something like this:

#!/usr/bin/perl -w
use strict; 

use XML::Twig;

XML::Twig->new( # process just list elements
                twig_roots => { list => \&process_list },
                # output the rest as is
                twig_print_outside_roots => 1,
              )
         ->parse( \*DATA);

sub process_list
  { my( $t, $list)= @_;
    # wrap (non-empty) lines in a listitem element
    my @listitems= $list->split( qr/(^.+)\n/m => 'listitem');
    # add the p and extract tags within each listitem
    foreach my $listitem (@listitems)
      { $listitem->insert( 'p', 'extract'); }
    $list->print ;
  }

__DATA__
<extract>
<line>
<p>
<extract>
<show>
<list>
first item
second item
third item
</list>
</show>
</extract>
</p>
</line>
</extract>
[download]

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