from HTML/Entity.pm:

# Make the opposite mapping
while (my($entity, $char) = each(%entity2char)) {
    $entity =~ s/;\z//;
    $char2entity{$char} = "&$entity;";
}
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so try: change %char2entity as well

package HTML::Entities;
$char2entity{'.25'} = '¼';
$char2entity{'.5'}  = '½';
$char2entity{'.75'} = '¾';

package main;
...
[download]

or post some code to look at.