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

my $literal_tag= "literal";

{ local undef $/;
  for (<DATA>)
    { # tag version, the && version would be even simpler
      # s{&&&(.*?)&&& }{xml_escape($1)}ges;
     s{<\s*$literal_tag\s*>(.*?)<\s*/\s*$literal_tag>}
       {xml_escape($1)}geso;
      print;
    }
}

sub xml_escape
  { my $literal= shift;
    $literal=~ s/&/&amp;/g;
    $literal=~ s/</&lt;/g;
    return $literal;
  }

__DATA__
<doc>
  <p>A regular para</p>
  <code><literal>there you put the code you 
        want, including & and <> and all</literal>