I just noticed there is more than one -expand=>1
Change the first one.

Here's more context:

my $ctl = $mw->Frame(-borderwidth => 1,
                     -relief => 'solid',
                     -width => 300,
                     -bg => 'aquamarine',
                    )->pack(
                            -side=>'left',
                            -fill=>'y', -expand=>0,
                            -anchor => 'w',
                           );
[download]