If you want to search for a pattern, then do what Corion advised---decompress the file; however, you can look at the gzip and count the lines.

#!/usr/bin/perl

use strict;
use warnings;

my $lines = 0;
my $filename = '/path/to/ImageMagick.tar.gz';
die "Can't open '${filename}': $!" unless open(FILE, '<', $filename);
while (sysread FILE, my $buffer, 4096) {
               $lines += ($buffer =~ tr/\n//);
}
print $lines, "\n";
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