I guessed the "problem" from just reading the description, hardly needed to look at the code. The problem comes from not closing the filehandle. Then the buffer isn't flushed, and hence the file is empty. You will never have more than one file suffering from this problem, and it's always the last file you wrote - this is because opening the same handle closes it first (if it was open). Good thing you didn't localize the handle!

One way of solving it is by using an autovivified scalar as handle:

    for (@files) {
        ... stuff ...
        open my $output => ">", $filename or die "....";
        print $output $built;
        push @built => [$filename => $_];
    }
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then when $output goes out of scope, that is, when the for block reaches the end, the file is automatically closed.

It is slightly better to do an explicit close yourself, as that allows you to inspect the return value of close. A close might fail (filesystem full, for instance).

-- Abigail