If you want wrapping done automatically for you, how about using a Text widget instead?:

my $textbox = $mw->Text( 
    -wrap => 'word',
)->pack( 
    -fill => 'both', 
    -expand => 1,
);

$mw->after(100 => sub { initialize() });
MainLoop;


sub initialize {
    $textbox->insert("1.0", $long_text);
}
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Then you can choose between the following types of wrap:

    -wrap => 'none' .... Don't do any wrapping at all
    -wrap => 'char' .... Wrap between characters
    -wrap => 'word' .... Wrap between words
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