Alternatively clone the whole thing.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=1023759
use warnings;

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

use Data::Dump 'dd'; dd 'before block', \@data;

LOCAL_BLOCK: 
  {
  use Storable qw( dclone );

  my @data = @{ dclone(\@data) };
  $_->{unit} = 'S' for @data;

  use Data::Dump 'dd'; dd 'using dclone in block', \@data;
  }

use Data::Dump 'dd'; dd 'after block', \@data;
[download]

Outputs:

(
  "before block",
  [
    { unit => "S", value => 50 },
    { unit => "T", value => 60 },
    { unit => "Q", value => 70 },
  ],
)
(
  "using dclone in block",
  [
    { unit => "S", value => 50 },
    { unit => "S", value => 60 },
    { unit => "S", value => 70 },
  ],
)
(
  "after block",
  [
    { unit => "S", value => 50 },
    { unit => "T", value => 60 },
    { unit => "Q", value => 70 },
  ],
)
[download]

---

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