use strict; use warnings; use Data::Dumper;

sub localised {
	package localised;
	my ($ref, $new) = @_;
	my $old = $$ref;
	$$ref = $new;
	return bless [$ref, $old];
	sub DESTROY {
		my ($ref, $old) = @{+shift};
		$$ref = $old;
	}
}

my @data = (
	{ unit   => 'S', value  => 50 },
	{ unit   => 'T', value  => 60 },
	{ unit   => 'Q', value  => 70 },
);

LOCAL_BLOCK_1: {
	my @guards = map localised(\$data[$_]{unit}, "S"), 0 .. $#data;
	print Dumper \@data;
}

# Note that @guards has gone out of scope.
# Hey, presto... DESTROY gets called!

print Dumper \@data;