Can be shortened to:

perl -le'$_=sprintf"%b",pop;s/../$&%8/eg;print'
[download]

Fails in case the binary expension has odd length. can be fixed like this for example:

perl -le'$_=sprintf"%b",pop;s/..(?=(..)*$)/$&%8/eg;print' 31
[download]

update (broquaint): added formatting