I see a lot of responses talking about this argument as the "mask" argument. That's the "mode" argument, or "permission mode" to be a little more verbose.

Perhaps the confusion is that the bits of the "umask" are cleared out of this value, per normal Unix policy on creating new files and directories. If it was a "mask", what is it masking? {grin}

And yes, this was mandatory in Perl 2 through Perl 5.5, and became optional in Perl 5.6.