First, you're missing the end-flush. Before you're really "DONE", you need to add $p->flush(). That gets the extra </foo> tag you're missing in your output. Not that you want it, but once you fix the other problem, you'll want it back.

Second, it's the twig_print_outside_roots flag that's doing it. Remove that. Instead, change your flush calls (including the new one) to have the param "$outfh". Now you'll flush to that file.

That leaves me with: 

#!/usr/bin/perl
use strict;
use warnings;
use XML::Twig;
open my $outfh, '>', "out.xml" or die ">out.xml:$!";

my $p = XML::Twig->new(
                       #twig_print_outside_roots => $outfh,
                       twig_roots => {
                           record => sub {
                               $_->set_text("altered ".$_->text);
                               shift->flush($outfh),
                           }
                       },
                       empty_tags => 'html',
                       keep_encoding => 1,
                       keep_spaces => 1,
                      );
$p->parsefile("in.xml");
$p->flush($outfh);
print "DONE\n";

[download]
As to why, ... I'm not sure.

Hope that helps,

Update: Ok, I see you really want the twig_print_outside_roots feature. It doesn't seem to do what you want it to, though. I am curious, though, as to why the formatting matters - this is XML, after all...

In reply to Re: XML::Twig outputting root element start tag twice by Tanktalus
in thread XML::Twig outputting root element start tag twice by benizi

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.