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$ wc words.txt
 113809  113809 1016714 words.txt

$ 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

use strict;
use warnings;
...
    $histogram{$key}++ if defined $val and $val eq 'Z';
}
print scalar keys %histogram;

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