$ wc words.txt
 113809  113809 1016714 words.txt

##</code><code>##

$ perl -ne 'chomp; for my $let (A..Z) { print  "$_ ". "foobar" x 200, " $let\n" }' words.txt > test_file.txt
$ wc test_file.txt
   2959034    8877102 3586152466 test_file.txt


##</code><code>##

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;

##</code><code>##

$ time perl test_Z_bigfile.pl
113809
real    0m13.973s
user    0m11.734s
sys     0m2.140s