use strict;
use warnings;
use Digest::MD5 qw/md5_hex/;

my $md5_hex_1m = file_md5_hex_1m('File.txt');
print $md5_hex_1m;

sub file_md5_hex_1m {
	my ( $file, $contents ) = @_;
	local $/ = \1_048_576;

	open my $fh, '<', $file or die $!;
	return Digest::MD5->new()->add($contents)->hexdigest
	  if defined( $contents = <$fh> );
}