oct EXPR
oct

Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to start off with 0x, interprets it as a hex string. If EXPR starts off with 0b, it is interpreted as a binary string. Leading whitespace is ignored in all three cases.) The following will handle decimal, binary, octal, and hex in the standard Perl or C notation:

$val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use sprintf() or printf():

$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;

The oct() function is commonly used when a string such as 644 needs to be converted into a file mode, for example. (Although perl will automatically convert strings into numbers as needed, this automatic conversion assumes base 10.)