Correct use of perl stack macros

Further to pull #49 this is the result of an audit of the use of perl
stack macros. There were several cases where PUTBACK was being called
where the stack pointer could be out-of-date.

Also some misc. cleanup where the macros were used needlessly.

src/perl/perl-core.c

@@ -58,9 +58,6 @@ static void perl_script_destroy_package(PERL_SCRIPT_REC *script)
 
 	perl_call_pv("Irssi::Core::destroy", G_VOID|G_EVAL|G_DISCARD);
 
-	SPAGAIN;
-
-	PUTBACK;
 	FREETMPS;
 	LEAVE;
 }