sub do_something {
    my ($red, $yellow, $blue) = @_;
    ...
}

##</code><code>##

my ($red, $blue, $green) = qw(1 1 1);

sub do_something {
    my ($red, $blue, $green) = @_;
    
    $$red += 1;
    $$green += 30;
}

do_something(\$red, \$blue, \$green);

print "$red, $blue, $green\n";

##</code><code>##

my %hash = (
    red => 1, 
    blue => 1, 
    green => 1, 
);

sub do_something {
    my $href = shift;

    $href->{red} += 1;
    $href->{green} += 30;
}

do_something(\%hash);

print "$hash{red}, $hash{blue}, $hash{green}\n";