use HTML::TreeBuilder::XPath;

# Parse html
my $tree = HTML::TreeBuilder::XPath->new;
my $parsed = $tree->parse_content( $some_html );

# Find some node
my $nodeset = $parsed->findnodes('//iframe');
for my $node (@$nodeset) {
    #### Would something like that be possible somehow? ###
    $tree->replace_by($node, $some_other_node);
}