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

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