#!/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;