To fix your current problem, change

-sticky => 'we',
[download]

to

-sticky => 'nsew',
[download]

See Tk::Pane for details.

You also have quite a lot of other issues. Here's half a dozen that seemed immediately obvious:

• my $mw = MainWindow->new(); is a fairly standard convention. By using $mw for a different window, your code becomes confusing.
• You have "our $mw = $mw_top->Scrolled(Pane, ...". Beyond the $mw issue (in the last point), why are you using our instead of my?
• -gridded => 'y' is meaningless. See Tk::Pane for the reason.
• $mw->Frame; is pointless: other than using some memory and CPU, it does nothing.
• $frame_commons_first_level and $frame_entry_info have no content.
• our $t: again, why are you using our instead of my?