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

use XML::Twig;

my $t = XML::Twig->new(
    twig_handlers => { b => \&b, },
    pretty_print => 'indented',
);
$t->parse( \*DATA);
$t->flush;

sub b {
    my ( $t, $b ) = @_;
    foreach my $c ($b->children('c'))
      { # yep, that does it: wrap a b element around the c
        $c->wrap_in( b => { name => $c->att( 'name') } ) 
      }
    
    $b->erase; # remove the original b
    $t->flush; # you need to flush here if you want to free the memory
}

__DATA__
<a>
<b name="funny words">
  <c name="foo"/>
  <c name="baz"/>
</b>
<b name="foods">
  <c name="apple"/>
  <c name="pear"/>
  <c name="cheese"/>
</b>
</a>