That's the "mode" argument, or "permission mode" to be a little more verbose.

I agree that the argument should be named "mode", but that's not how it is documented. In 5.8's perlfunc:

    mkdir FILENAME,MASK
    mkdir FILENAME

[download]

Note: in 5.005, it was called mode.

In reply to Re: •Re: "not enough arguments" with 'mkdir' command by jsprat
in thread "not enough arguments" with 'mkdir' command by nysus

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