Guards.

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;

[download]
package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

In reply to Re: Localizing hash keys in an AoH by tobyink
in thread Localizing hash keys in an AoH by kennethk

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