Looking at the documentation of XML::DOM, it shows the use of ->getElementsByTagName. The simplest approach would then be something like:

my $signals = $doc->getElementsByTagName('signal');
my @by_id = grep {
                my $sigid= $_->getAttributeNode('sigid');
                if( $sigid ) {
                    $sigid == 3464
                } else {
                    0  # no sigid attribute, no match
                }
            } map { $signals->item($_) } 0..$signals->getLength()-1
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I still recommend using an XPath query.