Hashes sound like the way forward

open(my $cities_fh => 'cities.txt') or die "ack: $!";
my %cities;
chomp, $cities{lc substr $_, 0, 3} = $_
  while <$cities_fh>;

open(my $devices_fh => 'devices.txt') or die "ack: $!";
/^dev-([a-z]+)/ and print "$cities{$1} $_"
  while <$devices_fh>;
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So create a hash with the keys as the first three letters of the city, lower-cased, and the values as the city & state name. Then iterate through the devices file grabbing the city identifier then printing out the city & state name along with the filename.