Just being picky here but...
use CGI;
But you didn't.
$/ = "\n";
All very well, but $/ will be this anyway, most people use "\n%%\n" as their cookie file delimiter so they can format them to 76 chars/line or whatever so as to fit them in emails as well.
print "Content-type: text/html\n\n\n";
But you're printing out a badly formed html document unless each and every cookie starts <html> etc...
A better solution would be:
#!/usr/bin/perl -w
use strict;
use CGI;
my $output;
my $data = '/path/to/cookies.txt';
my $query = CGI::new();
open COOKIES, "<$data" or die "Cannot open cookie file: $!";
$/ = "\n%%\n";
rand($.) < 1 && ($output = $_) while <COOKIES>;
# for plain text:
print $query->header('text/plain');
print $output;
# or for html
print $query->header('text/html');
print $query->start_html('Cookies');
print $output;
print $query->end_html;
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