roboticus:

Thanks for the explanation. As a test, I augmented the example I gave above with an equivalent function in C. (This was my first experience using Inline::C. To call the Perl sub from within C, I copied the example in the section “Calling Perl from C” of the Inline::C-Cookbook.)

#! perl
use strict;
use warnings;

$| = 1;

use Inline C => <<'END_C';

void baz(SV* foo_ref)
{
    printf("Begin baz()\n");
    fflush(stdin);
    dSP;
    ENTER;
    SAVETMPS;
    XPUSHs(sv_2mortal(newSVpvf("Plus an extra line")));
    PUTBACK;
    call_pv("destruct", G_DISCARD);
    printf("Before FREETMPS\n");
    fflush(stdin);
    FREETMPS;
    LEAVE;
    printf("-End- baz()\n");
    fflush(stdin);
}
END_C

{
    package Foo;

    sub new
    {
        my ($class, $num) = @_;
        my  $self = { id => $num, };
        return bless $self, $class;
    }

    sub DESTROY
    {
        my ($self) = @_;
        print 'Foo::DESTROY(', $self->{id}, ")\n";
    }
}

my  $foo1 = Foo->new('first');
my  $foo2 = Foo->new('second');
bar($foo1);
print "Back in main (1)\n";
baz($foo2);
print "Back in main (2)\n";

sub bar
{
    print "Begin bar()\n";
    destruct($_[0]);
    print "-End- bar()\n";
}

sub destruct
{
    undef $_[0];
}
[download]

Output:

22:55 >perl 552c_SoPW.pl
Begin bar()
Foo::DESTROY(first)
-End- bar()
Back in main (1)
Begin baz()
Foo::DESTROY(second)
Before FREETMPS
-End- baz()
Back in main (2)

22:56 >
[download]

From which it seems that the object is destroyed — meaning its reference count has fallen to zero — before the FREETMPS macro is reached. In which case the addition of the object to the sub’s call stack has not caused its reference count to be incremented? (Or am I simply misunderstanding here?)