use strict;
use XML::Parser;
use XML::Writer;

my $writer = XML::Writer->new();
my $parser = XML::Parser->new(
   Handlers => {
      Init  => \&handle_Init,
      Start => \&handle_Start,
      Char  => \&handle_Char,
      End   => \&handle_End,
      Final => \&handle_Final,
   }
);

# i could have also made these $parser attributes
# such as $parser->{from} and $parser->{to}
our $from = 'foo';
our $to   = 'struggle';

my $data  = do {local $/;<DATA>};
$parser->parse($data);

# called once at the beginning
sub handle_Init {
   $writer->xmlDecl('UTF-8');
   $writer->doctype('xml');
}

# called each time a start element is encountered
sub handle_Start {
   my($self,$name,%atts) = @_;
   $name = $to if $name eq $from;
   $writer->startTag($name,%atts);
}

# called each time non-markup data is encountered
sub handle_Char {
   my($self,$text) = @_;
   $writer->characters($text);
}

# called each time an end element is encountered
sub handle_End {
   my($self,$name) = @_;
   $name = $to if $name eq $from;
   $writer->endTag($name);
}

# called once at the end of the document
sub handle_Final {
   $writer->end();
}

__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xml>
<xml>
  <foo class="life or death">
    <opponent>wolf</opponent>
    <opponent>ant</opponent>
  </foo>
  <foo class="life or death">
    <opponent>pantomime goose</opponent>
    <opponent>Terrance Rattigan</opponent>
  </foo>
</xml>