... around -> ... where is the hidden side effect to use whitespaces there ??

Nowhere. It's purely a layout/formatting/readability issue. I myself would have no objection to a block of code like

$w   ->{'foo'  } = ...;
$xx  ->{'boff' } = ...;
$yyy ->{$j     } = ...;
$zzzz->{fun($n)} = ...;
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and often write stuff like that. OTOH, I would strongly object to something like
    my $v = $hr -> {w} -> {x} -> {y} -> {z};
simply because I can't read it! I hate "exploded" code unless it's in the context of multiple rows of expressions with the same or similar columnar organization.