... 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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My personal preference is no spaces around the arrow, and I guess that's what the majority does, too, even in C++. At work, though, I'm often confronted with code that has a tab on the left hand side of the arrow, and a space on the right hand side.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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It's not a syntactic limitation, it's one of style and convention. The convention seems to be not putting space around the arrow operator. However, there are times where it's appropriate. Have a look at Test::Mojo for an example of where using a little whitespace with the arrow operator turns out to be quite elegant (for some values of elegant).

Here is a preview...

 $t->websocket_ok('/echo')
    ->send_ok('hello')
    ->message_ok
    ->message_is('echo: hello')
    ->finish_ok;
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This works because many object methods in the Mojolicious world return a reference to their object instance, so you can chain calls one after the other in that fashion.

About the only other time that I'll add whitespace around a -> operator is when it makes most sense to break a long line there:

my $value = decode_json($json)
    ->{'foo'}[0]{'bar'};
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That's a contrived example, but occasionally putting a line break at the first arrow improves readability.

Great nice breath at the Monastery. Thank you all.