1. The session struct contains a pointer to a string. This string is passed from the Perl code. I am assuming that perl will free string so I do not specifically free the memory associated with the string.

This is right, basically, but it's still risky business.  As you have it, the string your C struct points to is the PV part¹ of a scalar variable, which might have gone out of scope (and thus the PV being freed for reuse, too) at the time some of your other routines is referencing it.  For example, consider the following case:

sub get_connection {
    # ...
    my $sess_id = "Session ID Goes Here";
    return MySession::new($sess_id);
} 
# $sess_id out of scope now

my $x = get_connection();
my ($p, $q, $str) = MySession::get($x);   # internally references PV p
+art of $sess_id
[download]

This might not lead to problems right away, because Perl's memory allocation often doesn't reuse the memory immediately, but there's no guarantee whatsoever that the respective memory location will continue to hold the same string content after the associated SV has gone out of scope.

To not run into potential problems, you'd have to make sure one of the following

• the respective scalar ($sess_id here) doesn't go out of scope, either by
• writing the Perl code such that it doesn't happen (easy to overlook, once you've forgotten the XS implementation details!), or
• incrementing the SV's reference count on the XS side (and decrementing it when you no longer need it)
• copy the string into newly allocated memory on the XS side, which you then free yourself

___

¹ to verify, dump $sess_id on the Perl side using Devel::Peek, and print the address of str on the C side (printf("addr of str: %p\n", str);).  You'll get something like

SV = PV(0x750b78) at 0x777ce8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7723c0 "Session ID Goes Here"\0
  CUR = 20
  LEN = 24

addr of str: 0x7723c0
[download]

Note the same address 0x7723c0.

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