I don't think substr is what he's looking for. That would chop off the end of the string, not pad it. But in the interest of benchmarking I ran the following.

use Benchmark;
my $str = 'hello';
timethese( 10000000, {
            'pack'      =>  q{  $str = pack("A10",$str);        },
            'sprintf'   =>  q{  $str = sprintf("%-10s",$str);   },
});
[download]

And got these results, not much of a difference.

Benchmark: timing 10000000 iterations of pack, sprintf...
      pack: 18 secs (16.86 usr  0.00 sys = 16.86 cpu)
   sprintf: 18 secs (17.36 usr  0.00 sys = 17.36 cpu)
[download]

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