my %dup; # duplicate charachters hash

print "<TABLE BORDER='1'>\n";
for (<DATA>) {
    chomp;
    next if /^\s*$/;
    my @elements = split", ",$_;        
    map{exists $dup{$_} ? $_= "&nbsp;" : $dup{$_}++ }@elements;
    print_row(@elements);
}
print "</TABLE>\n";

sub print_row {
    print"<TR>";
    print"<TD>$_</TD>" for @_;
    print"</TR>\n";
}


__DATA__
csc, tech, base
csc, comp, acm
csc, mous, base

##</code><code>##

# start by printing the opening table token
print "<TABLE BORDER='1'>\n";

# now lets iterate over the lines of data
foreach my $line(<DATA>) {
    # chomp of the \n from each line
    chomp($line);
    # ignore blank lines in the data
    next if $line =~ m/^\s*$/;
    # split the line into an array of elements
    my @elements = split", ",$line;
    # now iterate over the elements, if the element has 
    # already been seen then set it to &nbsp;
    # which is the HTML token for a space, otherwise
    # add the element to our duplicates hash
    # I did this using map as shown:        
    # map{exists $dup{$_} ? $_= "&nbsp;" : $dup{$_}++ }@elements;
    # in this context map is just a fancy way of 
    # writing a loop structure. 
    foreach (@elements) {
        # the magical $_ is assigned to each element
        # of our array @elements
        # the (condition) ? do this : (else) do that
        # construct is a one line version of the 
        # if (cond) {do this} else {do that} below
        # so the map has the effect of seven lines...
        if (exists $dup{$_}) {
            # $_ is magical -> by changing $_ 
            # we  change this element of @elements
            $_ = "&nbsp;";
        } else {
            # define a new key of our %dup hash
            # you can just do $dup{$_}++ to assign
            # a value of one to $dup{$_} but this 
            # is more obvious
            $dup{$_} = "seen, so define $_ key";
        }
    } 
    # print out the line via our sub, formating
    # each element for a HTML table  
    print_row(@elements);
}
print "</TABLE>\n";

sub print_row {
    print"<TR>";
    # @elements is passed to sub via @_ array, this
    # print"<TD>$_</TD>" for @_;
    # is just short form for:
    foreach my $element(@_) {
        print"<TD>$element</TD>"
    }
    print"</TR>\n";
}

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csc, tech, base
csc, comp, acm
csc, mous, base