#!/usr/bin/perl -wl
use strict;

my @numberarray   = qw(1 120 13); 
lengthy(@numberarray);

sub lengthy {
  s{
      ( ^ ) 
   }
   {
      ( "0" x (6-length $_) )
   }xe and print for @_;
}
__END__
##</code><code>##

perl -wle ' s/^/("0"x(6-length))/xe and print for @ARGV;' 133 121 10 13

##</code><code>##

perl -wle ' s/^(\d+)/("0"x(6-length).$1)/xe and print or
    s/^(\w)/("\x20"x(6-length).$1)/xe and print for @ARGV;' 133 121 10 13 red