Я бы использовал pQuery, но это будет работать
#!/usr/bin/perl --
use strict;
use warnings;
use HTML::TreeBuilder;
my $html = <<'__HTML__';
<div id="target">old <B>i</B><I>n</I>ner</div>
__HTML__
{
my $t = HTML::TreeBuilder->new_from_content($html);
print $t->as_HTML('<>&',' ',{}), "\n";
my $target = $t->look_down( id => 'target' );
$target->delete_content;
$target->push_content(
HTML::TreeBuilder->new_from_content(
"<B>NEW</B>"
)->look_down(qw!_tag body!)->detach_content
);
print $t->as_HTML('<>&',' ',{}), "\n";
}
__END__
<html>
<head>
</head>
<body>
<div id="target">old <b>i</b><i>n</i>ner</div>
</body>
</html>
<html>
<head>
</head>
<body>
<div id="target"><b>NEW</b></div>
</body>
</html>
Да, я RTFM