Here is a solution to your problem :

use strict;
use vars qw/%hash/; 
use Tk;
use Tk::TList;   
use Tk::HList;

my $mw = MainWindow->new();
my $image = $mw->Getimage('folder');
my $tlist = $mw->TList(-orient => 'vertical');
my $label = $mw->Label(-width=>15);
my $hlist = $mw->HList( -itemtype   => 'text',
                                         -separator  => '!',
                                         -selectmode => 'single',
                                         -browsecmd  => sub 
                                         {
                                             my $file = shift;
                                             $label->configure(-text=>
+$file);
                                         });
                                         
chomp (my @work_file=<DATA>);

for (@work_file)
{
   next if ($_ !~ m!,!);
   
   @data = split (/[,\s]+/,$_);

   $ref = \%hash;
   foreach (@data) {   
       $ref->{$_} = {} if (not defined $ref->{$_})       
       $ref = $ref->{$_};
   }   
}

foreach my $bu (keys %hash)
{
    $hlist->add($bu, -text=>$bu);
    
    foreach my $group (keys %{$hash{$bu}}) {
        $hlist->add("$bu!$group", -text => $group);
        
        foreach my $function (keys %{$hash{$bu}->{$group}}) {
            $hlist->add("$bu!$group!$function", -text => $function);
        }
    }    
}

$hlist->pack;
$label->pack;

MainLoop;


__DATA__
Dept            Group            Function
IT,            Projects,        SAN/NAS
IT,            Development,        GUI
IT,             Security,

HR,            Admin,            West    
HR,            Management,        West

Legal,            Admin,            FirmWide
Legal,             Compliance,
[download]

There are several things to say :

• Be careful of the hlist separator, which you chose to be '/', whereas some of your data contains such a character. In my script, I've chosen '!' to be the hlist separator. It is used to build the tree. The '/' is classical as it corresponds to a file system tree.
• I use 'use Tk;' to import the whole functions of the module.
• I use a hash reference to build a internal representation of your tree and then go through it to be the hlist

Some things should be improved though. For example, one could use only one pass to create the hlist, that is build the hash and the hlist is the same loop.

HTH