Thanks, Rob, this was really bad choice, on my part, of example data, and I appreciate you pointed this out.

With real script, I'm observing smaller matrix population (non-zero elements count) after feeding 64-bit data in, than expected, when running with 5.32 PDL-SP (2.6 GSL), compared to correct count (population equals items fed in) running with 5.26 PDL-SP (2.3 GSL).

Then I tried to present exaggerated examples, and, yes, you are right, I messed operator precedence, and so result looks silly and not trustworthy.

There's still wrong output of "1 1" with:

die if gsl_spmatrix_set( $m, 1, 0, 1 );
say gsl_spmatrix_nnz( $m );
die if gsl_spmatrix_set( $m, 1 + (1 << 32), 0, 1 );
say gsl_spmatrix_nnz( $m );
[download]

or similar. I mean, my blunder doesn't cancel out Math::GSL (and perhaps GSL) issues.

I messed operator precedence, and so result looks silly and not trustworthy

Oh, no worries - that happens. (I know ;-)
For me, irrespective of the values assigned in the script, both gsl-2.6 and gsl-2.7.1 produce the same results.
That would suggest that the problem lies with the recent versions of the gsl library.
But, of course, the way I built Math-GSL-0.43 against gsl-2.7.1 (along with the test failures) doesn't inspire complete confidence in it ;-(

Playing around with it a bit, I find there's something (very strangely) buggy for the index 1 + (1 << 32), with both gsl-2.6 and gsl-2.7.1.
Have a fiddle with this, and you'll see what I mean:

use strict;
use warnings;
use feature 'say';
use Alien::GSL;
use Math::GSL;
use Math::GSL::SparseMatrix ':all';

my $m = gsl_spmatrix_alloc( 1, 1 );

my $v;

#$v = 3 + (1 << 32); # no problem with this value
#$v = 2 + (1 << 32); # no problem with this value
#$v = 0 + (1 << 32); # no problem with this value
$v = 1 + (1 << 32); # blows up, but only if
                    # gsl_spmatrix_get($m, $v, 0)
                    # is called. 

die if gsl_spmatrix_set( $m, 1, 0, 1.5 );
say gsl_spmatrix_get($m, 1, 0);
say gsl_spmatrix_nnz( $m );
die if gsl_spmatrix_set( $m, $v, 0, 1.75 );
say gsl_spmatrix_get($m, $v, 0); # comment out to avoid blow up
                                  # when $v is 1 + (1 << 32).
                                  # But still gives wrong result
                                  # (for that value only).
say gsl_spmatrix_nnz( $m );
[download]

The first thing to try is to run that as a C program and see if the same issue arises.
If not, then it's an issue with Math::GSL.
It's getting late over here ... something for tomorrow.

Cheers,
Rob

... something for tomorrow.

It's apparently a problem with the gsl library:

/*
This file: gsl_spmatrix.c
Modify the following "-I..." and "-L..." arguments accordingly.

Using the gsl-2.6 library that ships with Strawberry Perl 5.32.1-PDL, 
+build with:
gcc -o gsl_spmatrix.exe gsl_spmatrix.c -ID:/sp/_64/sp-5.32.1-PDL/c/inc
+lude -LD:/sp/_64/sp-5.32.1-PDL/c/lib -lgsl -lgslcblas -lm

Then execute gsl_spmatrix.exe

Using the gsl-2.7.1 library provided by Alien, build with:
gcc -o gsl_spmatrix.exe gsl_spmatrix.c -ID:/sp/_64/sp-5.32.1/perl/site
+/lib/auto/share/dist/Alien-GSL/include -LD:/sp/_64/sp-5.32.1/perl/sit
+e/lib/auto/share/dist/Alien-GSL/lib -lgsl -lgslcblas -lm

Then execute gsl_spmatrix.exe
*/
#include <stdio.h>
#include <stdlib.h>
#include <gsl/gsl_spmatrix.h>

int main(void) {
 size_t v = 1;
 v <<= 32;
 v += 1;
 printf("size of size_t is %d bytes\n", sizeof(size_t));
 gsl_spmatrix *m = gsl_spmatrix_alloc(1,1);
 gsl_spmatrix_set( m, 1, 0, 1.5 );
 printf("1: got: %f\n", gsl_spmatrix_get( m, 1, 0));
 printf("1: nnz: %d\n", gsl_spmatrix_nnz( m ));
 gsl_spmatrix_set( m, v, 0, 1.75 );
 if (v >= m->size1) printf("will be reported as 'out of range'\n");
 printf("2: got: %f\n", gsl_spmatrix_get( m, v, 0 ));
 printf("2: nnz: %d\n", gsl_spmatrix_nnz( m ));
 return 0;
}


/*
As is, the built executable outputs (for both gsl-2.6 and gsl-2.7.1):

D:\C>gsl_spmatrix.exe
size of _size_t is 8 bytes
1: got: 1.500000
1: nnz: 1
will be reported as 'out of range'
gsl: getset_source.c:27: ERROR: first index out of range
Default GSL error handler invoked.

But if the line "v += 1;" is changed to "v += 2" or to "v += 3" or to 
+"v += 0" then the output is:

D:\C>gsl_spmatrix.exe
size of _size_t is 8 bytes
1: got: 1.500000
1: nnz: 1
2: got: 1.750000
2: nnz: 2
*/
[download]

I haven't yet reached an understanding of the problem.
I think it should be reported to the gsl developers unless that has already been done.

Cheers,
Rob