but bbb->PUNCT­UATION doesn't exist

Did you try it?

package aaa;
use constant HELLO=> 'world';
use constant PUNCTUATION=> '!';

package bbb;
our @ISA= qw( aaa );
use constant HELLO=> 'universe';

package main;

print "\n Hello ", aaa->HELLO(), aaa->PUNCTUATION();
print "\n Hello ", bbb->HELLO(), bbb->PUNCTUATION();
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produces:

 Hello world!
 Hello universe!
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