Surrounding the identifier with curly braces is more frequently seen in double-quoted strings to separate it from neighbouring text. For example,

"The time now is ${time}A.M."

This syntax also works outside strings, as you cited, but that is rarely needed and being most of the time redundant. It is there to support identifiers that start with a ^ sign, which is normally not allowed. However, I cannot think of any case that really needs to refer this kind of variables.

For more information, refer to

perlvar - perldoc.perl.org