$excel{$tab}{$row}{$col} = $cell->{Val};

...

foreach my $tab_name (keys %excel) {
   # Only intersted in "Customers" tab.
   next if $tab_name ne 'Customers';

   my $rows = $excel{$tab_name};

   foreach my $row_name (sort { $a <=> $b } keys %$rows) {
      my $cols = $rows->{$row_name};

      # Not interested in rows where the first cell is blank.
      next if !$cols->{A};

      ...[ Do something with %$cols ]...
   }
}