I scratched my head over this for a while. Very clever. Here's my attempt at explaining the key parts.

()=/(.*)/s,

$_ is an alias for one of the reversed strings in @_. Assuming this is being run by Perl 5, list context forces $1 to get set in the absence of /g. (perlre notes that $1 wouldn't get set in Perl 4.) $1 now refers to the entire string held in $_ (including the "\n" in the final one, thanks to /s).

$_=reverse,
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Use scalar context to set $_ to a reversed copy of itself.

print $1
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Since $1 still refers to the entirety of $_, the reversed string is printed. (You could just print $_ here, but that would hardly be obfuscating.)

for @_=(" tsuJ", ...)

@_ is a false clue. It could as well be @x. The array assignment makes a copy of the array. This permits individual elements (pointed to by $_) to be reversed. Without the array assignment here, reverse would fail in an attempt to reverse a read-only string.