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;
}