Well footnote 7 does say:

Some contexts allow 2 or even 1 digit, but any usage without exactly three digits, the first being a zero, may give unintended results. (For example, in a regular expression it may be confused with a backreference; see "Octal escapes" in perlrebackslash.)

So it would be much more clear with the 3 digit representation. But in this particular context, it works out:

use feature 'say';
use Data::Dump qw(dump);
say dump( pack("c*", 10, 3, 30, 21 ) ); # outputs "\n\3\36\25"
say dump( unpack("c*", "\n\3\36\25") );  # outputs (10, 3, 30, 21)
say dump( unpack("c*", "\n\003\036\025") );  # outputs (10, 3, 30, 21)
[download]

Ah, that was the necessary clue. Sometimes the perl interpreter is too clever for me. Many thanks.