Read my post carefully. The mode given to sysopen is not the file permission mode. The resulting file permission mode is the result of the mode given to sysopen, masked by the umask.

A file mode of 0644 or 0600 do not restrict the file owners options as the owner can still read/update/delete the file.

Read my post carefully. I said A mode of 0666 gives the user the option to say who can read/write the file, which is something entirely different from the permissions of the owner of a file.

Let me quote man perlopentut

       Permissions A la mode

       If you omit the MASK argument to "sysopen", Perl uses the
       octal value 0666.  The normal MASK to use for executables
       and directories should be 0777, and for anything else,
       0666.

       Why so permissive?  Well, it isn't really.  The MASK will
       be modified by your process's current "umask".  A umask is
       a number representing disabled permissions bits; that is,
       bits that will not be turned on in the created files' per­
       missions field.

       For example, if your "umask" were 027, then the 020 part
       would disable the group from writing, and the 007 part
       would disable others from reading, writing, or executing.
       Under these conditions, passing "sysopen" 0666 would cre­
       ate a file with mode 0640, since "0666 & ~027" is 0640.

       You should seldom use the MASK argument to "sysopen()".
       That takes away the user's freedom to choose what permis­
       sion new files will have.  Denying choice is almost always
       a bad thing.  One exception would be for cases where sen­
       sitive or private data is being stored, such as with mail
       folders, cookie files, and internal temporary files.
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Abigail