I think it's mostly a question of style.

The &-thingy has the disadvantage that beginners leave out the parentheses and wonder why the subroutine gets the arguments of its parents; otoh this is an advantage too, because you can't do the same with the arrow notation (and you can't do goto &$sub with arrows either). The arrow notation can sometimes be shorter, because you may not need the braces around an expression, or you might even leave out the arrow in newer versions of perl, for example:

perl -we '$op{"+"} = sub { $_[0] + $_[1] }; $op{"-"} = sub { $_[0] - $
+_[1] }; print $op{$ARGV[1]}($ARGV[0], $ARGV[2]), $/;' 8 - 2
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Otoh, when you do need parentheses around the expression generating the function, the ampersand-notation is shorter.

Other than these minor points, the difference is aesthetiyc. I always use the ampersand notation, moreover, I never use the arrow for dereferencing either an array or a hash or a code. I do use the arrow for method calls of course. (This style is new, it wasn't like this in my earlier scripts.)