The SvREFCNT macro (perlguts) returns this number. I am sure this is already somewhere on cpan, but a simple XS routine could look like this:

/* generated by h2xs -c Scalar::RefCount */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"



MODULE = Scalar::RefCount       PACKAGE = Scalar::RefCount

int
refcount(s)
    SV* s
PROTOTYPE: $
CODE:
    RETVAL = SvREFCNT(s);
OUTPUT:
    RETVAL
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use Test::More tests => 6;
BEGIN { use_ok('Scalar::RefCount') };

#########################

my $a;

is(Scalar::RefCount::refcount($a),1, 'return 1');

my $b = \$a;
is(Scalar::RefCount::refcount($a),2, 'return 2');

my $c = \$a;
is(Scalar::RefCount::refcount($a),3, 'return 3');

{
my $d = \$a;
is(Scalar::RefCount::refcount($a),4, 'return 4');
}
is(Scalar::RefCount::refcount($a),3, 'return 3');
[download]

Update: Ah, thanks cdarke.