This "CALLBACK" is a so-called bareword and in certain places (e.g. here when used as hash-key) barewords are implicitely quoted.

So $hash{CODEREF} and $hash{"CODEREF"} are the same thing.

Another typical usage would be 

my %hash = (
             HUBBA => 1,
             BUBBA => 2
           );

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Again the keys to the hash are implicitely quoted.

However you have to be careful when you use non-chars in the key: $hash{HUBBA-BUBBA} will not work as the Perl-parser parses the key as an expression and you get an error (you would have to quote explicitely here: $hash{"HUBBA-BUBBA"}.

The use of all-caps is only a matter of taste and has no significance.

In reply to Re: beginner question - why is hash key unquoted and in capitals by morgon
in thread beginner question - why is hash key unquoted and in capitals by Anonymous Monk

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