Looks like a bug introduced in the post-0.77 trunk:
Good:
$ perl -le 'use Net::DNS; print Net::DNS::RR->new(
name=>"x.com",type=>"TXT",txtdata => ["foo","00","bar"])->string'
x.com. 0 IN TXT foo 00 bar
Wrong:
$ perl -le 'use Net::DNS; print Net::DNS::RR->new(
name=>"x.com",type=>"TXT",txtdata => ["foo","0","bar"])->string'
x.com. 0 IN TXT ( foo
bar )