You can get a substantial improvement over the builtin sprintf with: 
  substr "000$num", -3

[download]
  
use strict;
use Benchmark qw(cmpthese);

my $num = 42;
cmpthese (-3, {
  sprintf => sub { sprintf '%03d', $num },
  concat  => sub { ('0' x (3 - length $num)).$num },
  substr  => sub { substr "000$num", -3 },
});

            Rate  concat sprintf  substr
concat  244047/s      --    -10%    -36%
sprintf 269956/s     11%      --    -29%
substr  381628/s     56%     41%      --

[download]
   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

In reply to (MeowChow) Re5: Help with number conversion by MeowChow
in thread Help with number conversion by Mr.T

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