#!/usr/local/bin/perl

use Benchmark;


my $var=13;
my $length=6;

timethese(100000, {
 CONCAT=>\&concat,
 SPRINTF=>\&spr_f,
});

sub concat {
 my $a = $var;
 while (length($a)<$length) {$a="0$a";}
}

sub spr_f {
 my $a = $var;
 sprintf("%0$length", $a);
}

Results:
Benchmark: timing 100000 iterations of CONCAT, SPRINTF...
    CONCAT:  2 wallclock secs ( 1.53 usr +  0.00 sys =  1.53 CPU) @ 65359.48/s (n=100000)
   SPRINTF:  0 wallclock secs ( 0.60 usr +  0.00 sys =  0.60 CPU) @ 166666.67/s (n=100000)