Writing your own solution sounds like a good golf problem.

Assuming you have a valid 9 digit mode string with -'s where the bits are off, here's a small snippet that'll usually do what you want:

$_ = "-r-x-w-rwx"; # Mode string in $_
my $bits = 0;
for (split //) {
    $bits *= 2;
    $_ ne "-" and $bits++;
}
printf "My bits are: %o\n", $bits;
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Not completely size optimized, but perhaps a bit easier to understand this way.

The basic technique is to loop through the string, checking each character to see whether it's a "-" (off bit) or not (on bit). Each time through the loop, multiply $bits by 2 to shift the previous result out of the way, and then add 1 if the lowest bit should be turned on.

This does not detect the setuid or setgid bits, or anything other than standard drwxrwxrwx bits correctly.

Alan