${var} and $var are the same thing in an interpolation. People use the first form if the next thing after it is an identifier: 

say "${var}foo"; # interpolates $var, adds literal foo
say "$varfoo";   # interpolates $varfoo

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So in your case there's no reason to write it, but it seems the author deemed the form with curlies safer.

Perl 6 - second systems done right

In reply to Re: what is the use of derefernce in substitution? by moritz
in thread what is the use of derefernce in substitution? by tej

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