You can add an index to XPath queries if you want the nth element:

//parent/a[@class="nav"][4]
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Note that the following does not work because numbering is according to parent/ancestor, not in the results:

//a[@class="nav"][4]
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If you don't have a suitable common ancestor, you will have to find the element by using the text() function or extract the element using Perl:

my @nav= $mech->xpath('//a[@class="nav"]');
$mech->click( $nav[3] );
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