The "mask" argument to perlfunc:mkdir must be an *octal number*,

No, it must be a numeric expression. We commonly express the number as octal, because those octal numbers have mnemonic value. But if you can calculate in your head that 0777 is the same as {firing up calculator...} 511 decimal, then just type 511 there, and it works just fine.

The problem with writing 777 there is that it's interpreted as if it was 01411, which is interpreted by Unix as sticky bit, read-only for user, execute-only for group and others. Certainly not a good permission for a new directory.

But let's kill this meme that this second parameter "must be octal". It must be a number, and we usually write it as octal because of the mnemonic value.

-- Randal L. Schwartz, Perl hacker