In your inner loop you must not use absolute pathes but relative like this:

use strict;
use warnings;
use XML::LibXML;

foreach my $filename (@ARGV) {
  my $parser = XML::LibXML->new();
  my $doc    = $parser->parse_file($filename);
  foreach my $person ($doc->findnodes('/edgarSubmission/relatedPersons
+List/relatedPersonInfo/relatedPersonName')){ 
    print $_->data . " " for $person->findnodes('firstName/text()');  
+  
    print $_->data . " " for $person->findnodes('lastName/text()');   
+ 
    print "\n";
  }
}
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