use strict;
use warnings qw( all );

sub fuse { ... }

my $o1 = { voltage => 1 };
my $o2 = { voltage => 2 };
my $o3 = { voltage => 3 };
my $o4 = { voltage => 4 };
say join " ", map $_->{voltage}, $o1, $o2, $o3, $o4;  # 1 2 3 4

fuse(\$o2, \$o1);
fuse(\$o3, \$o4);
fuse(\$o3, \$o1);
say join " ", map $_->{voltage}, $o1, $o2, $o3, $o4;  # 4 4 4 4

$o1->{voltage} = 5;
say join " ", map $_->{voltage}, $o1, $o2, $o3, $o4;  # 5 5 5 5

$o3->{voltage} = 6;
say join " ", map $_->{voltage}, $o1, $o2, $o3, $o4;  # 6 6 6 6