You are referencing the hash to $s, which is local.
You could pass in a ref to $sid, or return the hash ref. I prefer the later.

Also you are assigning a hash ref to a hash, which is probably what you don't want. {} creates a hash reference, () are used to define a hash.

One of these 2 options should work:

#Passing scalar ref
sub make_session {
    my ($u,$a,$s)=@_;
    my $session_hash={
        user=>$u,
        dept=>$a
    };
    $$s=$session_hash;
}
&make_session($user,$dept,\$sid);
[download]

or

#Returning hash ref
sub make_session {
    my ($u,$a)=@_;
    $session_hash={
        user=>$u,
        dept=>$a
    };
    return $session_hash;
}
$sid = &make_session($user,$dept);
[download]

Update: Changed %session_hash to $session_hash after realizing that you are creating a hash ref by using {}.