There are some advantages to working in-place over creating a temporary file:

• File permission and ownership is maintained.
• Using a temporary file requires more permissions.
• Using a temporary file may complicate the locking system required.
• Working in-place can be faster in some situations.

This is pretty close to what the -i commandline switch does for Perl one liners

-i is closer to

open(my $fh_in,  '<', $qfn);
unlink($qfn);
open(my $fh_out, '>', $qfn);
...
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for now.