in reply to How to add label?

G'day aydar.pl,

Welcome to the Monastery.

You're dereferencing $main_window in

my $label_of_telephone_or_email = $$main_window->new_ttk__label(...

Just use $main_window directly, i.e. remove the extraneous '$':

my $label_of_telephone_or_email = $main_window->new_ttk__label(...

and the GUI runs normally.

— Ken