References
See http://perldoc.perl.org/perlreftut.html
my %combination;
$combination{25} = 0;
$combination{10} = 0;
$combination{ 5} = 0;
$combination{ 1} = 0;
$combination{25} += 3;
my $cref = \%combination;
$cref->{25} -= 1;
say Dumper(%combination);
Dereference a list
my @list = (25, 20, 5, 1);
my $list_ref = \@list;
for my $element (@{$list_ref})
{
say $element;
}