If you define HEAD as a constant, then you cannot change it anymore because it is, indeed, a constant. "ABC" is a string and it is its first element.
If you first define the constant ABC and then the constant HEAD, then you can do the following:

use strict;
use warnings;

use constant ABC=> qw/wet ear rtf/;
use constant HEAD => ABC,qw/DEF GHI/;
print $_ foreach(HEAD);
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