When you localize the array, it's only the array elements that get localized; you still have the original hash refs.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

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

LOCAL_BLOCK: {
    local(@data) = @data;
    $_->{unit} = 'S' for @data;
}

print Dumper \@data;
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outputs

$VAR1 = [
          {
            'unit' => 'S',
            'value' => 50
          },
          {
            'unit' => 'S',
            'value' => 60
          },
          {
            'unit' => 'S',
            'value' => 70
          }
        ];
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