I think that what's happening to you is that your command is missing -e EXPR. Without EXPR, the command will start to run and forget to finish, slowing things down until the whole computer grinds to a halt.

The trick to zgrep is that it really isn't very portable. For example, the -d option isn't supported by my system's grep. Here's what I used that worked:

#!/usr/bin/perl

use strict;
use warnings;

my $file = shift @ARGV;

open(INFILE, "|-", "zgrep -c -e / $file ") or die $!;
close INFILE;
[download]

You'll notice that I used "|-": it writes to STDOUT, which is what I think that you were trying to do. I also tried it with "-|": it writes to STDIN, and that didn't work.