From the perlguts manpage:

       There are some convenience functions available that 
       can help with the destruction of xVs.  These
       functions introduce the concept of "mortality".  An
       xV that is mortal has had its reference count marked
       to be decremented, but not actually decremented,
       until "a short time later".  Generally the term
       "short time later" means a single Perl statement,
       such as a call to an XSUB function.  The actual
       determinant for when mortal xVs have their reference
       count decremented depends on two macros, SAVETMPS and
       FREETMPS.  See perlcall and perlxs for more details
       on these macros.

       "Mortalization" then is at its simplest a deferred
       "SvREFCNT_dec".  However, if you mortalize a variable
       twice, the reference count will later be decremented
       twice.

       "Mortal" SVs are mainly used for SVs that are placed
       on perl's stack.  For example an SV which is created
       just to pass a number to a called sub is made mortal
       to have it cleaned up automatically when it's popped
       off the stack. Similarly, results returned by XSUBs
       (which are pushed on the stack) are often made
       mortal.

Flo