Generaly, session files look something like this:

$D = {
    "_SESSION_EXPIRE_LIST" => {},
    "_SESSION_REMOTE_ADDR" => "127.0.0.1",
    "_SESSION_ATIME" => "1057334715",
    "_SESSION_CTIME" => "1057334688",
    "_SESSION_ID" => "2edc038f30ed95d3bd43f113b5385a5d",
    "_SESSION_ETIME" => undef
};
[download]

(Note, I've added some formatting. It's usually just one long line.)

As you can see it's a dumped (see Data::Dumper) data structure. It starts off with an assignment to $D. When using strict you have to declare your variables. Thus my $D; before the eval.

By eval()ing, I'm executing that code, thus doing the assignment to $D.

Check this out:

use strict;
local $/;

my $D;
eval( <DATA> );

print $D->{ _SESSION_ID };

__DATA__
$D = {
    "_SESSION_EXPIRE_LIST" => {},
    "_SESSION_REMOTE_ADDR" => "127.0.0.1",
    "_SESSION_ATIME" => "1057334715",
    "_SESSION_CTIME" => "1057334688",
    "_SESSION_ID" => "2edc038f30ed95d3bd43f113b5385a5d",
    "_SESSION_ETIME" => undef
};
[download]

Try commenting out my $D;. You'll get an error.

HTH