I have put together a script to compare user input with a db and want to extract the bits of the input which do not match the db contents

This is very convoluted and I feel is crying out for a hash but I'm not sure how to create the hash or what to do with it once I've got it!!

Having put this together I feel sure that if a kindly monk would show me a more efficient way then it would be really useful to me in terms of improving my skills

Thanks

#comma delimited user input
@words = split(/,/, $words);
#copy of array for later
@words2 = @words;



open FH, "$data" or die "Can't open $data: $!";
flock (FH, 1) or die "Can't lock $data for reading: $!";
my @all = <FH>;
close FH;

#iterate through user input comparing with db
while (@words) {
my $item = shift (@words);
$count++;


    foreach my $line (@all) {
    ($one,$two,undef,$four,$spare_1,$spare_2) = split "\t",$line;
        chomp;
        if ($item eq "$four") { 
        push (@matched, $item);
        push (@counted, $count);
        last;
        }
    }
}

#iterate through @counted applying undef to matched items in copy of u
+ser input array
while (@counted) {
    my $item = shift (@counted);
    $words2[$item -1] = undef;
    }

#remove undef items from user input leaving only unmatched items.
foreach my $item (@words2) {
    unless ($item eq "") {
        push (@uniq, $item);
    }
}        

#@matched contains only matched items
#@uniq contains only unmatched items
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