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

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

LOCAL_BLOCK: {
    my $cmd;
    $cmd .= "local \$data[$_]{unit} = 'S';\n" for 0 .. $#data;
    $cmd .= 'print Dumper \@data';
    eval $cmd;
}

print Dumper \@data;