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# Pass by value:
    $obj= My::Class->new( %hash );
    $obj= My::Class=>new( key=>"value", key2=>$value2 );
# vs. pass by reference:
    $obj= My::Class->new( \%hash );
    $obj= My::Class=>new( { key=>"value", key2=>$value2 } );

sub new {
    my( $class, $hashRef )= @_;
    my $self= bless $hashRef, $class;
    return $self;
}

    my %hash= ( key=>"value", key2=>$value2 );
    my $obj1= My::Class->new( \%hash );
    $hash{key}= "cheap";   # This changes $obj1!
    my $obj2= My::Class->new( \%hash );

sub new {
    my( $class, $hashRef )= @_;
    my $self= bless {%$hashRef}, $class;
...
    return $self;
}
# etc.