Well, if you only ever have "r", "w", "x", and "-" present, then you can use pack/unpack for this:

sub perm2mode {
    my( $perm )= @_;
    if(  9 != length($perm)
     ||  9 != ( $perm =~ tr/rwx-/1110/ )  ) {
        die "Unsupported permission string ($_[0])";
    }
    return unpack "v", pack "b*", ''.reverse$perm;
}
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Use sprintf "0%o" on the return value if you want an octal string, but don't try to use that octal string as a number or you'll get the wrong number:

my $mode= perm2mode("rw-r----x");
my $str= sprintf "0%o", $mode;
print "mode=$mode str=$str\n";
my $bad= 0+$str;
printf "bad=%d=0%o mode=%d=0%o\n", $bad, $bad, $mode, $mode;
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produces:

mode=417 str=0641
bad=641=01201 mode=417=0641
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So, for example, using chmod $str, $file; would be wrong just like chmod "0660", $file; is.

        - tye (but my friends call me "Tye")