Code:
my $oBook = $Excel->Workbooks->Open("C:\\Inetpub\\wwwroot\\YAI\\WIN_TEST\\INPUT\\abc.xls"); 
$Excel->{DisplayAlerts}=0;


my $sheetcnt = $oBook->Worksheets->Count();
print "---Total wks=$sheetcnt---\n";

# Block 1 works fine
foreach  my $iSheet(1..$sheetcnt){
 my $name=$oBook->Worksheets($iSheet)->{Name};
 my $Sheet = $oBook->Worksheets($iSheet);
print "At Pos=$iSheet Wks name=$name\n";

  if($name=~/Header/){$H_TEST{"HEADER"}{$iSheet}=1;}
  if(($name=~/ALO/)){$H_TEST{"DATA"}{$iSheet}=1;}
}

print "\n-------------------------\n";
# Block 2 works fine
foreach my $Sheet(in $oBook->{Worksheets}){
   print "\t" .$Sheet->{Name} ."\n";
}
print "\n-------------------------\n";

# Prints the content of the hash - works fine
foreach my $tag(sort keys %H_TEST)
{
foreach  my $iSheet(sort keys %{$H_TEST{$tag}}){
    print "Tag=$tag at $iSheet\n";
}
}

print "\n-------------------------\n";

# ---> This block does not work. Particularly
 #  my $sheet = $oBook->Worksheets->{$iSheet};
 # If $iSheet is replaced with numeric value it works

foreach my $tag(sort keys %H_TEST)
{
foreach  my $iSheet(sort keys %{$H_TEST{$tag}}){   
 my $sheet = $oBook->Worksheets->{$iSheet};
 my $name=$oBook->Worksheets($iSheet)->{Name};

print "Tag=$tag Position2=$iSheet and name=$name\n";
}
}

---------------Output with errors--------

Content-type: text/plain

---Total wks=3---
At Pos=1 Wks name=Header
At Pos=2 Wks name=SCM_NAME_RULES
At Pos=3 Wks name=ALO

-------------------------
	Header
	SCM_NAME_RULES
	ALO

-------------------------
Tag=DATA at 3
Tag=HEADER at 1

-------------------------
<h1>Software error:</h1>
<pre>Win32::OLE(0.1707) error 0x80020009: &quot;Exception occurred&quot;
    in METHOD/PROPERTYGET &quot;3&quot; at C:\Inetpub\wwwroot\YAI\TESTING\DB\read_excel_cells.pl line 99
</pre>
<p>
For help, please send mail to this site's webmaster, giving this error message 
and the time and date of the error.

</p>
[Fri Oct 19 11:07:43 2007] read_excel_cells.pl: Win32::OLE(0.1707) error 0x80020009: "Exception occurred"
[Fri Oct 19 11:07:43 2007] read_excel_cells.pl:     in METHOD/PROPERTYGET "3" at C:\Inetpub\wwwroot\YAI\TESTING\DB\read_excel_cells.pl line 99