If you replace 'H1' by 'Heading 1', the resulting data will no longer be well-formed XML, tag names cannot include spaces, so are you sure you want to do this?

Looking at your data, I am also not sure if the names are right. It looks to me like you have nested sections, unless you have quite complex headings, that include a title, text and sub-headings with more text. So maybe something like section1 and section2 would be more appropriate.

So with a proper XML parser (I am partial to XML::Twig), which makes the solution easier to write and more robust:

#!/usr/bin/perl

use strict;
use warnings;
use XML::Twig;

XML::Twig->new( twig_handlers => { H1 => sub { $_->set_tag( 'section1'
+); },
                                   H2 => sub { $_->set_tag( 'section2'
+); },
                                 },
                pretty_print => 'indented',
              )
         ->parsefile( $ARGV[0])
         ->print;
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