my $hold = readfile('holds');
  my $copy = readfile('copies');
  for my $key (keys %$hold) {
    if ($copy{$key}) {
      print "$key: hold and copy (or copy and hold)\n";
    }
  }

  sub readfile {
    my $file = shift;
    my $hash = {};
    open(my $fh, "<$file") or die "$file: $!";
    local $_;
    while (<$fh>) {
      # fields are '|' delimited - pick up the first field
      my $key = substr $_, 0, index($_, "|");
      ++$hash->{$key};
    }
    close $fh;
    return $hash;
  }