As for your actual objective:

use HTML::Entities qw( decode_entities );

sub my_decode_entities {
   local $HTML::Entities::entity2char{egrave} = chr(232);
   return decode_entities(@_);
}
[download]

Another way:

use HTML::Entities qw( _decode_entities );

{
   my %entity2char = (
      %HTML::Entities::entity2char,
      egrave => chr(232),
   );

   sub my_decode_entities {
      return map { _decode_entities($_, \%entity2char } @_;
   }
}
[download]

In both cases, you don't break anything (such as HTML generating functions) by modifying the hashes since the scope of the modifications has been limited.

By the way, I wouldn't call it a "default" list. It should be comprehensive. If any HTML entities are not included, you should submit a bug report.