my @KEYWORDS = qw(hello there you gurus); 

# create the regex
my $string = '';
foreach (@KEYWORDS){
    $string .= "$_||";
}
$string =~ s/\|\|$//;

# open the log file
open(LOGFILE,$logfilename)||die"Cannot open $logfilename\n$!\n"

# let's go through the files....
foreach (@files) {
    $filename = $_;
    open(FILE,$filename)||die"Cannot open $filename\n$!\n";
    #check if a keyword is in the line
    while(<FILE>){
        if (/$string/i){
            print LOGFILE "Keyword found in $filename\n";
        }
    }
}