Perhaps pass "\0' rather than undef: 
my $foo = undef;
my $bar = wrap_create_bar(\$foo);

sub wrap_create_bar {
    if(defined(${$_[0]}) {create_bar($_[0]}
    else {create_bar(\"\0")}
}

[download]

Update: I've had a couple of goes at correcting the above code (which is, of course, untested). I hope I've got it right ... or at least made my intention clear.

I'm basing that on the following Inline::C script, for which the foo1() sub produces the warning, but the foo2() sub does not. (Plus the fact that C seems to regard "\0" as equivalent to NULL.) 
use warnings;
use Inline C => Config => BUILD_NOISY => 1;

use Inline C => <<'EOC';

void foo1(SV *x) {
     printf("%s\n", SvPV_nolen(x));
}

EOC

$x = undef;
foo1($x);
foo2($x);

sub foo2 {
    if(defined($x)) {foo1($x)}
    else {foo1("\0")}
}

[download]

The problem is that as soon as a perl API function detects that it has been passed an undef, you'll get that warning (iff warnings are enabled). Simplest solution is to therefore pass a value that is not regarded as undef, but still has the desired effect - which means modifying the undef argument appropriately before it gets passed to the XSub. At least that's the only approach I could get to work for me ... though there may be others.

Cheers,
Rob

In reply to Re: Passing NULL pointer through XS without "Use of uninitialized value in subroutine entry" by syphilis
in thread Passing NULL pointer through XS without "Use of uninitialized value in subroutine entry" by kscaldef

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