use strict;   # Please
use warnings; # Please

# $n = 0;  # There may be a better way

# 'my' added to localise the scope of these variables
# It won't matter much now, but when you get to large programs its a life saver
my $logz = '/var/log/syslog';     
my $file = '/home/user/Desktop/keywords';     

# Nowadays we use the three parameter form of open.
# its good to check the results too.
# open(LOGFILE, $logz);   
# open(KEYWORD, $file);   
open my $log, '<', $logz or die "Can't read $logz: $!\n";   
open my $keys,'<', $file or die "Can't read $file: $!\n";

# I guess you want to check the keys against each logline
# here I am going to take a different approach

# Dont read the log all in one go, we will just check it line
# at a time
# @keyWord = <$keys>;
# @logFile = <LOGFILE>;

# precompile the regex for the keys
my %re_key; # regex compiled keys
while (my $pattern = <$keys> ) { # check each key
    chomp $pattern; # remove line ending
    $re_key{$pattern} = qr/$pattern/; # store in a hash
}

# $keyz = @keyWord[$n]; # this was only looking at one key. 

while (my $line = <$log> ) {
    for my $pattern (keys %re_key) {
        print "$pattern matched in $log" if $line =~ /$re_key{$pattern}/;
    }
}