One typo in your code might cause the problem:

$workbook = Spreadsheet::WriteExcel>new('FAISTATS_'."$Month".'.xls');
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should be

$workbook = Spreadsheet::WriteExcel->new('FAISTATS_'."$Month".'.xls');
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When I corrected this and ran it under use strict; (a few declarations are needed...), it created a file with the desired sheets.

use warnings;
use strict;
use Spreadsheet::WriteExcel;
use Spreadsheet::ParseExcel;
use Spreadsheet::ParseExcel::SaveParser;

my $Month = 9;
my @s;

sub create_excel {
  my $workbook = Spreadsheet::WriteExcel->new('FAISTATS_'."$Month".'.x
+ls');
  
  for ( my$i=1;$i<=8;$i++) {
    $s[$i] = $workbook->add_worksheet( "rpzea0".$i."a001" ) ;
  } 
}

create_excel();
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