It is mentioned in the manual. {} is just an alternative syntax to [] §:
Both square brackets and curly braces can be used interchangeably for accessing array elements (e.g. $array[42] and $array{42} will both do the same thing in the example above).
The same goes the strings §:
Characters within strings may be accessed and modified by specifying the zero-based offset of the desired character after the string using square array brackets, as in $str[42]. Think of a string as an array of characters for this purpose. [...]
Note: Strings may also be accessed using braces, as in $str{42}, for the same purpose.