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    for(my $iSheet=0; $iSheet < $oBook->{SheetCount}; $iSheet++)
    {
        print "$oBook->{SheetCount}";
    }

use strict;
use warnings;
...
        $var = $iSheet;
    }
}

Benchmark: timing 10000 iterations of For_C, For_Set...
     For_C: 14 wallclock secs (14.33 CPU) @ 697.84/s (n=10000)
...
          Rate   For_C For_Set
For_C    698/s      --    -50%
For_Set 1385/s     98%      --

$iSheet < $oBook->{SheetCount} # hash look up and comparison