$x = { a => 1 };   # a reference
$y = $x;           # a copy of a reference points to the same thing
$y->{a}++;         # increments $x->{a}
$y->{b} = 3;       # adds a hash item $x->{b}
$y = { c => 0 };   # breaks the connection between $x and $y
$y->{c}++;         # no effect on $x