You want to write a benchmarking script to test it..

*Untested, off the top of my head, code follows..

#!/usr/bin/perl

use Benchmark;

timethese(1000, {
   'perl' => \&perl_grep,
   'system' => \&system_grep,
});

sub perl_grep{
  open(OUTPUT, "$script | grep EXPRESSION |");
  my @output = <OUTPUT>;
}

sub system_grep{
  open(OUTPUT, "$script |");
  my @output = <OUTPUT>;
  @output = grep { EPRESSION } @output;
}
[download]

This will run each sub 1000 times and give you stats on their speed

cheers,

J