in reply to Need help with binding event in Tk

perldoc perldiag
Can't modify non-lvalue subroutine call

(F) Subroutines meant to be used in lvalue context should be declared as such, see Lvalue subroutines in the perlsub manpage.
So whatever is giving you the error is trying to use whatever as an lvalue and whatever is not meant to be used as one. Tk::Error is reporting on it.

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