It got me too (see Re: Re: Re: Re: random elements from array - new twist)... It's a matter of precedence. Have a look at how perl parses it:

perl -MO=Deparse,-p -e "$a = $_ + rand @{$r} - $_"
($a = ($_ + rand((@{$r;} - $_))));
                ^^           ^^
-e syntax OK
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The '- $_' is part of the argument to rand, not the assignment to $a.