People seem to be misunderstanding the point of my question. If you look at this code, you can see that there is a single line that may optionally be executed: the line that assigns $book to the static variable $closure_workbook. It's not worth explaining why, but I want to store $book in a static variable. And I see no reason that the one extra assignment should cause an error. 
use strict;
use Excel::Writer::XLSX;

&func(@ARGV);
{
    my $closure_workbook;
    
    sub func {
        my $file = 'perl.xlsx';
        my $book = Excel::Writer::XLSX->new($file);
        if (@_) {
            $closure_workbook = $book;
        }
        my $worksheet = $book->add_worksheet();
        $worksheet->write(0, 0, 'Hi Excel!');
    }
}

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In reply to Re^2: variable scopes in Excel::Writer::XLSX by johnrcomeau
in thread variable scopes in Excel::Writer::XLSX by johnrcomeau

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