/(?:Hello|world|today)/

##</code><code>##

/(?:Hello|world|today)/i

##</code><code>##

/(?:\bHello\b|\bworld\b|\btoday\b)/i

##</code><code>##

use strict;
use warnings;
use autodie;

my @arr;
my $logz = '/var/log/syslog';
my $file = '/home/user/Desktop/keywords';

open my $keysFH, '<', $file;
while (<$keysFH>) {
	chomp;
	push @arr, "\\b\Q$_\E\\b";
}
close $keysFH;

my $words = '(?:' . ( join '|', @arr ) . ')';
my $regex = qr/$words/i;

open my $logFH, '<', $logz;
while (<$logFH>) {
	print if /$regex/;
}
close $logFH;