use strict;
use XML::Twig;
my $file = $ARGV[0];

my $first = 'paragraph';
my $replace = 'p';

my $twig = XML::Twig->new(twig_handlers => 
{"$first" => sub {$_->set_gi("$replace")}},
);
$twig->parsefile($file);
$twig->print;