From perlop (my added emphasis):

Perl variable names may also be a sequence of digits or a single punctuation or control character. These names are all reserved for special uses by Perl; for example, the all‐digits names are used to hold data captured by back references after a regular expression match...

Finally, new in Perl 5.6, Perl variable names may be alphanumeric strings that begin with control characters (or better yet, a caret). These variables must be written in the form "${^Foo}"; the braces are not optional. "${^Foo}" denotes the scalar variable whose name is a con­trol−"F" followed by two "o"’s. These variables are reserved for future special uses by Perl, except for the ones that begin with "^_" (control−underscore or caret−underscore). No control‐character name that begins with "^_" will acquire a special meaning in any future version of Perl; such names may therefore be used safely.