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sub double {
    sub { \@_ }->($_[0], $_[0])
...
@$r; # => ( 2, 2 );
$r->[0] = 3;
@$r; # => ( 3, 3 );

switch_var $r, $v, my $w;
$w = 4;
@$r; # => ( 4, 4 );

sub switch_var { $_[0] = double $_[2] }

sub switch_var {
    alias $_[1] = $_[2];
...
switch_var $r, $v, my $w;
$w = 4;
@$r; # => ( 3, 3 );

sub switch_var {
    alias $_[0][0] = $_[2];
...
switch_var $r, $v, my $w;
$w = 4;
@$r; # => ( 4, 3 )

my $x;
my $fake_sub = fake_subroutine $x => sub { \@_ }->($x, $x);
...
my $r = call $fake_sub, my $y;
$y = 1;
@$r; # => ( 1, 1 );