If I'm understanding properly, you could do anything to the keys in this setting, because they're just strings and you can't get references to them to allow their modification, which is how the values get modified.

That's what my test code suggests, anyway:

#!/usr/local/bin/perl -w

use strict;

my $ref={ a => 'aa', b => 'bb'};

print "The hash, before:$/";
print %$ref;

print qq($/The hash "during":$/);
for (%$ref) {
    /[A-Z]/ or $_ = 'x';
    print;
}

print "$/The hash, after:$/";
print %$ref;
print $/;
__END__
The hash, before:
aaabbb
The hash "during":
xxxx
The hash, after:
axbx
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