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

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

# Works
LOCAL_BLOCK_1: {
    local $data[0]{unit} = 'S';
    local $data[1]{unit} = 'S';
    local $data[2]{unit} = 'S';
    
    print Dumper \@data;
}

print Dumper \@data;