The keys of (non-magical) hashes are always strings. Therefore, $iSheet contains a string, the stringification of the sheet number. Because $iSheet contains a string, you're telling Excel to return the Worksheet *named* "3" instead of the Worksheet *at index* 3. Convert $iSheet to a number.

my $sheet = $oBook->Worksheets->{0+$iSheet};
[download]

Better yet, use a HoA instead of a HoH. You never use the hash value, and I don't think you're using it to get rid of dups.

HoH (current):

if ($name =~ /Header/) { $H_TEST{"HEADER"}{$iSheet} = 1; }
if ($name =~ /ALO/)    { $H_TEST{"DATA"}{$iSheet}   = 1; }
...
foreach (sort keys %{$H_TEST{$tag}}) {
   my $iSheet = 0 + $_;
[download]

HoA (proposed):

if ($name =~ /Header/) { push @{$H_TEST{"HEADER"}}, $iSheet; }
if ($name =~ /ALO/)    { push @{$H_TEST{"DATA"}},   $iSheet; }
...
foreach my $iSheet (sort @{$H_TEST{$tag}}) {
[download]