IMHO it's not just about forgetting the comma. When you write it with or without comma, two very different things look very similar.

The additional curlies add "visual weight", one sees immediately that 

print $a, $b;
# and
print { $a } $b;

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do different things.

I don't usually use the curlies, but that's more a result from laziness and thoughtlessness than anything else.

I kinda like the Perl 6 way of doing it: 

print $a, $b;   # print both variables
$a.print: $b;   # treat $a as a file handle

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In reply to Re^2: curly braces around a lexical file handle by moritz
in thread curly braces around a lexical file handle by rastoboy

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