You could try Sys::Mmap. This seems to work for me:

#!/usr/bin/perl

use strict;
use warnings;

use Fcntl qw(:DEFAULT O_ASYNC O_DIRECT);
use Sys::Mmap;

sysopen(my $FH,"/dev/mem", O_RDONLY, 0666)
  or die "Couldn't open /dev/mem\n";
my $mem;
mmap($mem,1_000_000_000,PROT_READ,MAP_SHARED,$FH)
  or die "Couldn't mmap\n";

print "Billionth byte of memory: ",substr($mem,999_999_999,1),"\n";
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