In addition to updating the screen, which you can sprinkle liberally through your code if need be, you may also want to consider linking your textbox or label to a variable (a global works well in this case-- and since it's a GUI that's probably OK) rather than hardcoding the text in with configure.

my $Status = 'Starting application';
my $label = $main->label( -textvariable => \$Status );
$Status = 'Done loading application';
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You may still have to update the window to get changes in $Status to appear, but it will make updating a status bar a lot simpler to type.