The square brackets create a brand new array out of nothing--an array without a name--and return a reference to it. By putting @tooldescs in brackets you end up with a reference to a new, anonymous array containing @tooldescs' elements.

You can take a reference to @tooldescs itself using the backslash operator and give -msg that instead:

$bloon->attach( $toollist->Subwidget("scrolled"),
        -balloonposition => 'mouse',
        -msg => \@tooldescs );
[download]

but be forewarned that attach() can now modify @tooldescs directly. If that's OK, then you don't need the brackets, and you can get away with using the backslash. If it isn't, then use the brackets.

See perlreftut and perlref for more.