There are actually two things happening here that cause this code to require 5.6.

The prototype for unpack wants the first argument (the format) to be a scalar value. The first argument is actually evaluated in a scalar context.

@args = ('a5', 'qwert');
unpack @args;                       # not okay; equivalent to unpack 2
+;
unpack $args[0], @args[1..$#args];  # okay, format arg followed by dat
+a args
[download]

The reason why Anonymous Monk's code works in 5.6 but not in 5.005 is due to a change in the implementation of qw//. As you know, qw/STRING/ is equivalent to split ' ', 'STRING'. In 5.005, the split is performed at runtime, but in 5.6, the split occurs at compile time! Thus, given this code: unpack qw/a5 qwert/; perl5.005 compiles it as: unpack split ' ', 'a5 qwert'; which puts split in a scalar context, and leaves unpack with only one argument.
perl5.6, on the other hand, compiles it as: unpack 'a5', 'qwert'; and unpack is happy.

In conclusion, here's a version of Anonymous Monk's JAPH which runs in earlier versions of perl:

($#=unpack u, q&>>7,@2D%U='ET96YR;VAY96P@(%!R>6-E2')A:RP*&)
=~s:y(.)(.)(.)(.)(.)(.):\3\5\1\6\2\4:g;           print 42;
[download]