The first warning is simple. sprintf is a function that returns a formatted string. Yet you simple discarded that string. That makes no sense, so you got a warning. Perhaps you wanted printf?

"Numerical context" is a fancy way of saying the argument needs to be a number, and it will be coerced into one if necessary. It's usually very obvious when a number is required. For example, the arguments of addition must be numbers, and so does the argument for %u. Unfortunately, ^R doesn't look anything like a number, so a warning is issued ("isn't a number") and it's treated as zero. You seem to want the character's ordinal value.

printf('%u', ord($foo));
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Perhaps you're from a C background where everything is a number. In Perl, it's more like everything is a string.

                       #  C     Perl
                       #  ----  ----
printf('%u', 4);       #     4     4
printf('%u', '4');     #    52     4
printf('%u', 0x12);    #    18    18
printf('%u', '\x12');  #    18     0*

* - warns "isn't numeric"
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