The only thing that I see from looking at your code that might be responsible would be that

my($dev) = $devinfo->findnodes('./dev_name');
[download]

actually finds many nodes just to throw them all away except for the first. Maybe do it this way to check how many nodes you find:

my @found_devices = $devinfo->findnodes('./dev_name');
warn "Found " . scalar(@found_devices);
my $dev = $found_devices[0];
[download]

But that's just a shot in the dark. Actually looking at your XML, it shouldn't find more than one device.