$str = "&lt;pre><br>\n&lt;/pre>";

while($str =~ /(&lt;pre>.*?&lt;\/pre>)/gsi) {
      my $pos  = pos $str;
      substr($str, pos($str) - length($1), length($1)) =~ s/<BR>\n/\n/gsi;
      pos($str) = $pos;
}
print $str;