Because the form ${^X} must have an uppercase charater there. This comes from toke.c:

/* In variables name $^X, these are the legal values for X.  
 * 1999-02-27 mjd-perl-patch@plover.com */
#define isCONTROLVAR(x) (isUPPER(x) || strchr("[\\]^_?",(x)))
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And the ${^Xyzpdq} variables are an extension of those, so claims perlvar:

It understands `^X' (caret `X') to mean the control-`X' character. For example, the notation `$^W' (dollar-sign caret `W') is the scalar variable whose name is the single character control-`W'. This is better than typing a literal control-`W' into your program.

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. ...