Are you the file's user owner?
redirecting to a file that already exists preserves its owner
compare the files user owner with your euid.

    warn "WARN: not my file: $file" unless $> eq (stat($file))[4];
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If that doesn't help. Make sure that your not writing to
an unfriendly NFS or the like.
(from a UNIX shell) type: df -F nfs $file

Good luck,
Malus