In the source (and the ref you provided mentions this) of Mojo::Util html_entities.txt is hardcoded. From this file's contents %Mojo::Util::ENTITIES is constructed. So, it looks to me that without changing the filename in Mojo::Util you will not achieve what you want.

However, this being Perl and this being Perlmonks, here is a workaround which should be applied to all script parts which use Mojo::Util;. This may not be surely is not practical.

Mojo::Util provides the wonderfully named monkey_patch() which you can use to alter the only sub therein which uses %ENTITIES, which is Mojo::Util::_entity. And make it use your own (EDIT: e.g. _entity2 which is identical to the original but it uses your locally defined %ENTITIES which you create as you see fit):

use Mojo::Util;
my $str = '<foo>bar<baz>&"';
my $res1 = Mojo::Util::html_unescape($str);

my %ENTITIES = (lt => '<<<', gt => '>>>');

Mojo::Util::monkey_patch 'Mojo::Util', _entity => \&_entity2;

my $res2 = Mojo::Util::html_unescape($str);

print "res1: $res1\n";
print "res2: $res2\n";


sub _entity2 {
  my ($point, $name, $attr) = @_;

  print "_entity2 called...\n";

  # Code point
  return chr($point !~ /^x/ ? $point : hex $point) unless defined $nam
+e;
  # Named character reference
  my $rest = my $last = '';
  while (length $name) {
    return $ENTITIES{$name} . reverse $rest
      if exists $ENTITIES{$name} && (!$attr || $name =~ /;$/ || $last 
+!~ /[A-Za-z0-9=]/);
    $rest .= $last = chop $name;
  }
  return '&' . reverse $rest;
}
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bw, bliako