Variables declared with my inside a block { .. } are not accessible outside the block. You probably want something like

sub get_xpath_value {
   my ($xpath) = @_;   
   my @nodes = $root->findnodes($xpath);
   
   my ($name,$value); # declare outside block
   if (defined $nodes[0]) {
     $value = $nodes[0]->text; # still accessible inside block
     $name  = $nodes[0]->getName;
   } else {
     $value = 'not defined'; # or whatever you want
     $name  = 'not defined';
   }
   return ($name,$value); 
}
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