As you said, I indeed expect to get in the volumes key a data or a hash ref (and not both). I had a bug in the program so I added the print lines, and then I got what I got, this is the source of my bug but I can't understand how I can get such a thing.
The following is the full function (with debugs):

sub readClusterVolumes {

    my %volumesInfo;

    print "checkpoint 1:\n", Dumper(\%volumesInfo);
    print "vol: ", $volumesInfo{volumes}, ', ', $volumesInfo{volumes}{
+0}, ', ', $volumesInfo{volumes}{1}, "\n";
    my @volumesNodeIDs = &getParam($PARAMS_VOLUMES_NODE_LIST);  # shou
+ld return (0 1) or nothing dependin whether the parameter exists
    my (%volumes, $tmp, $volumeKey);
  
    $volumesInfo{maxVolId} = &getParam($PARAMS_MAX_VOLUME_ID);
    $volumesInfo{isInternal} = &getParam($PARAMS_INTERNAL_DISKS);

    for (my $i = 0; $i < @volumesNodeIDs; $i++) {
        print "in for loop\n";
        ($volumeKey = $PARAMS_NODEX_VOLUMES) =~ s/X/$volumesNodeIDs[$i
+]/;
        $tmp = &getCDBParam($volumeKey, $TRUE);
        if ($tmp && $tmp ne '') {
            print "in if, tmp :$tmp\n";
            $volumes{$volumesNodeIDs[$i]} = $tmp;
        }
    }
    print "checkpoint 2:\n", Dumper(\%volumesInfo);
    print "vol: ", $volumesInfo{volumes}, ', ', $volumesInfo{volumes}{
+0}, ', ', $volumesInfo{volumes}{1}, "\n";
    if (scalar(keys(%volumes)) == 0) {
        print "no voluems #######\n";
        $volumesInfo{volumes} = 'none';
    } else {
        print "voluems exist #######\n";
        $volumesInfo{volumes} = \%volumes;
    }
    print "checkpoint 3:\n", Dumper(\%volumesInfo);
    print "vol: ", $volumesInfo{volumes}, ', ', $volumesInfo{volumes}{
+0}, ', ', $volumesInfo{volumes}{1}, "\n";
    return \%volumesInfo;
};
[download]

I will also add that this happens in the second call to the function. in the first call the output is:

checkpoint 1:
$VAR1 = {};
vol: , ,
in for loop
in if, tmp :0 1 2 3 4
in for loop
in if, tmp :0 1 2 3 4
checkpoint 2:
$VAR1 = {
          'volumes' => {},
          'maxVolId' => '4',
          'isInternal' => 'false'
        };
vol: HASH(0x8479e1c), ,
voluems exist #######
checkpoint 3:
$VAR1 = {
          'volumes' => {
                         '1' => '0 1 2 3 4',
                         '0' => '0 1 2 3 4'
                       },
          'maxVolId' => '4',
          'isInternal' => 'false'
        };
vol: HASH(0x88d8144), 0 1 2 3 4, 0 1 2 3 4
[download]

and in the second call I get:

checkpoint 1:
$VAR1 = {};
Use of uninitialized value in print at /vobs/mng/exadmin/var/www/cgi-b
+in/lib/clusterEdit.pl line 21, <SESS> line 60.
Use of uninitialized value in print at /vobs/mng/exadmin/var/www/cgi-b
+in/lib/clusterEdit.pl line 21, <SESS> line 60.
Use of uninitialized value in print at /vobs/mng/exadmin/var/www/cgi-b
+in/lib/clusterEdit.pl line 21, <SESS> line 60.
vol: , ,
checkpoint 2:
$VAR1 = {
          'volumes' => {},
          'maxVolId' => 'none',
          'isInternal' => 'false'
        };
vol: HASH(0x847a2c0), ,
no voluems #######
checkpoint 3:
$VAR1 = {
          'volumes' => 'none',
          'maxVolId' => 'none',
          'isInternal' => 'false'
        };
vol: none, 0 1 2 3 4, 0 1 2 3 4
[download]

I removed some Unintialized value... warnings I got, so it will be more clearer. You can see that it somehow 'remembers' the old values from the first call (the last line of the second output). I can't understand how that can be.
All I want under the volumes key is none where there are no volumes, and the volumes themselvs when volumes exist.

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