I can't replicate your error message (tried with Tk version 804.028 and 804.027).

Without a selected entry I get "Use of uninitialized value in string ne at ./899028.pl line 30.", which could be fixed by testing 

    if(defined $lst->curselection)
    {
    ...

[download]

In reply to Re: Can't call method "curselection" on an undefined value by Eliya
in thread Can't call method "curselection" on an undefined value by Araviz

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