Maybe I'm missing something, but won't something like this work:

/* --- PerlVersionDemo.xs --- */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <stdio.h>


void print_hello_561 (void)
{
    printf("hello world from perl 5.6.1\n");
}

void print_hello_58 (void)
{
    printf("hello world from perl 5.8 or higher\n");
}

MODULE = PerlVersionDemo        PACKAGE = PerlVersionDemo    

#if (PERL_REVISION == 5) && (PERL_VERSION == 6) && (PERL_SUBVERSION ==
+ 1)

void
print_hello()
  CODE:
      print_hello_561();

#else
#  if (PERL_REVISION == 5) && (PERL_VERSION >=8)

  void
  print_hello()
    CODE:
        print_hello_58();

#  endif
#endif
[download]

Just stick your PROTOTYPE in the perl 5.8.x bit and it will be ignored on earlier perls.

Update: So, your share_561() would accept a regular variable and pass a reference to the actual share()

-- simonflk