The $mode value is 33261. I wanted to show what happens when you print it with the octal formatting string (prints 100755) and when you do it after having modified it with the binary and (&). And & has a higher precedence than the comma. As you can see below, adding parens to guarantee the right precedence does not modify the output::

  DB<1> $mode = 33261;

  DB<2> printf "%04o", $mode;
100755
  DB<3> printf "%04o", $mode & 07777;
0755
  DB<4> printf "%04o", ($mode & 07777);
0755
  DB<5>
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