Variation on a theme, with a twist that I don't think has been mentioned. I've tried to do my homework, and nothing stood out that would break this. What do you think?

#!/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__
[download]

Or, for the commandline:
perl -wle ' s/^/("0"x(6-length))/xe and print for @ARGV;' 133 121 10 13
Or, to make sure that it doesn't zero-pad alphabetical input
perl -wle ' s{(^)(\d+$)}{("0"x(6-length).$2)}xe and print for @ARGV;' 1 fish 002 fish 333 blue fish
And, to print all the arguments, but zero-padding only the numerical arguments:
perl -wle ' s{(^)(\d+$)}{("0"x(6-length).$2)}xe and print or print for @ARGV;' 1 fish 002 fish 333 blue fish

(Sir...put your hands up and please, step away from the keyboard)
One more.. to pad words with spaces, instead of zeros.

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
[download]

mkmcconn
(I'm gratified by how much better it is than this a few short weeks later. Thanks Perl Monks.).