The only place I see "getAttribute" is in this sub:

sub get_data {
    my($element, $attribute, $index) = @_;
    if ($index){$index=1;}

    my $nodeset = $xp->find("//yweather:$element");
    my $node = $nodeset->get_node($index);

    return $node->getAttribute($attribute);
}
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The error message means that $node is undef at this point. That means that undef was returned from $nodeset->get_node($index). I see a lot of calls to this sub, but in every case, there's no $index passed. According to the code that's there, that makes $index undef also. That's what I can see without running it.

My guess is that the bit that says "if ($index){$index=1;}" should actually say "if (!$index){$index=1;}" (note the exclamation point). That's just a guess.

To really find out what's going on, I'd recommend running this under the Perl debugger. See perldebug for the gory details.