You would do better to put the values in a hash, like this:

my %xml_values = ();
$xml_values{year} = '1999';

sub writeBasicTag {
    my $tag = $_[0];
    $writer->startTag($tag);
    $writer->characters($xml_values{$tag}); 
    $writer->endTag($tag);
}
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There are all sorts of articles around about why it's a bad idea to access variables by name through other variables.