local $_ = '...#18abcdefghi...';

/
   \#
   (\d)
   ( (??{ "\\d{$^N}"    }) )
   ( (??{ "(?s:.{$^N})" }) )
/x
   or die "No match\n";

print("$1:$2:$3\n");  # 1:8:abcdefgh