#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Memory::Usage;

my $file = shift;
open my $FH, '<', $file or die $!;

my $string;
my $mu = 'Memory::Usage'->new;

$mu->record('start');
{ local $/; $string = <$FH> };
$mu->record('after do');
undef $string;

seek $FH, 0, 0;
$string = do { local $/; <$FH> };
$mu->record('after no do');

$mu->dump;