sub createHoAoHfromAoHbyIndexingByTheValueOfOneKey{
    my( $AoHref, $key ) = @_;

    ### About 20 lines of parameter checking and error reporting/exception 
    ### handling here. Because unlike doing it in-line, the programmer writing 
    ### this function cannot know what the caller will pass him.

    my %HoAoH;
    push @{ $HoAoH{ delete $_->{ $key } } }, $_ for @$AoHref;
    return \%HoAoH;
}