To quote the HTML::Element documentation:

In list context, returns a list of elements at or under $h that have any of the specified tag names. In scalar context, returns the first (in pre-order traversal of the tree) such element found, or undef if none.

So, likely, the following code finds no element:

@stocks[$i]->find('a');
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Maybe you want the following code?

my $stock_url = @stocks[$i]->find('a')
    or die "Couldn't find a link in " . @stocks[$i]->as_HTML;
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As an aside, you better write it as

$stocks[$i]->find('a');
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which Perl would also recommend, if you were running with warnings enabled.