Something like this?

use File::Slurp;
my $file_content = File::Slurp::read_file('file');
$file_content = reverse $file_content;
my $memory_file = \$file_content;
open my $memfh, '<', $memory_file or die "$!\n";

#...seek
#...read

close $memfh;
[download]

For details see these articles from the FAQ:
read-in-an-entire-file-all-at-once and
open-a-filehandle-to-a-string