It strikes me that curlies (or braces) are used for legacy reasons given that the provenance of the language is the *NIX shell (see, to name but one, Perl).

As we all know, *NIX shell uses braces in references to environment variables - when and only when the variable isn't followed by a printable, non-word boundary char can the braces be omitted e.g. ${foo}_bar is the var foo with _bar appended, whereas $foo_bar refers to a variable named foo_bar.

This (string interpolation) is true for both shell and perl, but interestingly (I really ought to get out more:-) not in make(1) scripts - which can take braces or parens as separators.

Update:

Thanx to tilly for the hint.