I don't know about a completely general HTML solution because I am not an HTML expert. However, it could be that something simple would work ok? Here is some code that stops printing <nav sections after it has seen the first one. You could adapt this to your desired nth parameter functionality.
use strict;
use warnings;
my $nav_seen =0;
while (<DATA>)
{
# if inside of <nav> section, print it
# unless we have seen a <nav> section before
if (my $status = /<nav/ ... /<\/nav/)
{
print unless $nav_seen;
$nav_seen++ if $status =~ /E/;
}
else {print}
}
=PRINTOUT
<body>
<nav a=b>
<div>
</div>
</nav>
<div>
</div>
</body>
=cut
__DATA__
<body>
<nav a=b>
<div>
</div>
</nav>
<div>
</div>
<nav c=d>
<li> </li>
</nav>
</body>
To understand how this works, I direct you to
Flipin good, or a total flop?.