d0, d2 and d3 are the option numbers 208, 210 and 211 (in hex). Next byte in hex is the length of the option string. To convert "http://raidtest.anvin.org/tftpboot/" into that hex representation, you could e.g.

print join ":", map { sprintf "%x", ord $_ } split //, "http://raidtes
+t.anvin.org/tftpboot/";
[download]

See ord, hex, split, join, sprintf and map. Counting bytes with len and putting together the whole thing is left as an exercise to the reader. See also Net::DHCP::Options.