# your code is:
our $cb = $frame->new_ttk__button(-text => "Calculate"); 
our $filename = $cb->getOpenFile();
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I don't use Tkx, but from basic knowledge of buttons and callbacks, you don't have a callback for the button. Your code should look something like this:

our $cb = $frame->new_ttk__button(
               -text => "Calculate",  
               -command => sub{ our $filename = $cb->getOpenFile(); }
                );
[download]

See Tkx tutorial