$elt->first_child('LINK_CONFIG') returns an element, unless there is no LINK_CONFIG child. So if the code fails it will be at runtime, not compilation.

If this is likely to happen then you have to process this case:

if( my $link_config= $elt->first_child('LINK_CONFIG'))
  { my $relationship= $link_config->first_child('RELATIONSHIP')->text;
+ 
    # ...
  }
else
  { warn "no LINK_CONFIG found\n"; # or any other processing necessary
  }
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