use strict;
use warnings;
use HTML::TreeBuilder;

my $str = <<DOC;
<s1>King of England
JAMES, by the Grace of God, King of England, Scotland, France and Ireland,
Defender of the Faith... ... This first selection was written in England to
establish the colony of Virginia
<sb><s1>Rights of Landowners We, greatly commending, and graciously accepting
of, their Desires for the Furtherance</sb>
DOC

my $root = HTML::TreeBuilder->new ();

$root->ignore_unknown (0);
$root->parse ($str);
$root->eof ();

my @s1Nodes = $root->look_down ('_tag', 's1');

for my $node (@s1Nodes) {
     if ($node->look_up ('_tag', 'sb')) {
          # nested in a sb element - convert to h1
          $node->{_tag} = 'h1';
     } else {
          $node->{_tag} = 'section1';
     }
}

print $root->as_HTML ();