curselection problem in list box

[Vivek]

I am trying following code to see the selections made in current
listbox but can't see any output from print statement...

$io_dir_o = $frame2->Listbox(-selectmode => "multiple")->pack(-side =>
'left');
$io_dir_o->insert('end',qw/bidi out in od_bidi od_out analog avdd avss
vdd vss vddo corner filler/);
$io_dir_o->bind("<Button-1>");
$sel = $io_dir_o->curselection;
print (",$sel,");

Any suggestions. I am getting a ",," as output means empty string.
Thanks,
Vivek