package 'MathML::Entities::Approximate';
use MathML::Entities (); # explicit no-arg stops import
my %ENTITIES = (
    foo => '&x000042',
);

sub _convert2numbered {
    my $reference = shift;
    $reference =~ /^&([a-zA-Z0-9]+);$/;
    my $name = $1;
    # above copied from MathML::Entities
    return exists $ENTITIES{$name} ?
        $ENTITIES{$name} :
        MathML::Entities::_convert2numbered($reference);
}
# etc.