# Attempt to lock only $data_dir/search.idx, if allowed
if ($file_locking ne "No")
{
        flock (SIDX, LOCK_SH) or die "Can't set lock for ".  
                "file: $data_dir/search.idx: $!";
}

# Read whole files in to arrays.  This is a really bad 
# thing to do if the files are potentially huge.  If 
# they are small (< 1000 lines), however this is okay.
my @sidx = <SIDX>;
my @sidx2 = <SIDX2>;

# Closing filehandles removes the locks
close (SIDX);
close (SIDX2);

# Make sure that the search files have the same lengths (I'm 
# guessing here, perhaps this isn't important to you).
unless(@sidx = @sidx2) {
        die "Search files are of differing lengths.";
}

# Remove empty strings from @skeyw and @premiumkeyw
@skeyw = grep {$_ ne ""} @skeyw;
@premiumkeyw = grep {$_ ne ""} @premiumkeyw;

# Build a big regular expression out of @skeyw and 
# @premiumkeyw for faster matching.
my $regexp = join('|', @skeyw, @premiumkeyw);
$regexp = qr/$regexp/;

foreach my $line (@sidx) {
        my $premiumline = unshift(@sidx2);  # destructive.

        # In the previous code you write:
        # $sline = $line, $premiumline;
        # this is the same as:
        # $sline = $line;
        # I'm going to guess that you mean the following:
        $sline = "$line, $premiumline";

        # Do our test to see if this matches our regexp
        if($sline =~ /$regexp/) {
               
                # do something with search results

                $resultline[$icnt] = $line;
                $icnt++;
        }
}