The curlies are optional. Their only effect is to save semi-colons at the end of the assignments to $_.

Without the curlies, the following is left:

$_ = 'Just another Perl hacker';
$_ .= $/;
print sort reverse chr ord
   for split //;
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Since $_ contains only one character, chr ord is a no-op.
Since reverse is called in list context with a single argument, it's a no-op.
Since sort is called in list context with a single argument, it's a no-op.