use strict;
use warnings;
use feature qw/say/;

my %histogram;
open my $IN, "<", "test_file.txt" or die "could not open input file $!";
while (<$IN>) {
    chomp;
    my ($key, undef, $val) = split / /, $_;
    $histogram{$key}++ if defined $val and $val eq 'Z';
}
print scalar keys %histogram;