Altering the Return Value: Postwrappers

• The postwrapper can see the function's original return value

• This is quite simple:

 $return = &$original;
 $dummy = scalar $wrapper{post}->(@_, $return)
 if $wrapper{post};

• Altering the return value is simple, but subtle:

 return $return;

• Variables in Perl function calls are aliased inside the called function

 sub set_to_57 { $_[0] = 57 }
 my $x = 119;
 set_to_57($x);
 print "$x\n"; # prints 57!!

• Assigning to $_[-1] inside of the postwrapper modifies $return!