gistfile1.pl

    my @dest_airports = split ',', $hotel->{airports};
    
    my $counter = 0;

    for my $airport ( @filtered_dest_airports ) {
        if ( !$has_flights_cache{$airport} ) {
            $counter++;
            next;
        }
        # ...
        # code here
        # ...
    }
	
    print "filtered out $counter airports without flights";

###### versus: #####

    my @dest_airports = split ',', $hotel->{airports};
    my @filtered_dest_airports = grep $has_flights_cache{$_}, @dest_airports;
	
    print "filtered out ".( @dest_airports - @filtered_dest_airports )." airports without flights";

    for my $airport ( @filtered_dest_airports ) {
        # ...
        # code here
        # ...
    }