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sub opening_port
{
...

close($serial);
}

sub open_port
{
if($listbox1->curselection) 
...
{
&portwarning;
}

$btn1=$mw->Button(-text=>"Open",-width=>5,-height=>2,-command=>\&open_
+port)
        ->place(-relx=>0.05,-rely=>0.25);
$btn2=$mw->Button(-text=>"Close",-width=>5,-height=>2,-command=>\&clos
+e_port)
        ->place(-relx=>0.15,-rely=>0.25);