You might find something in perlform. The following is close but puts the extra whitespace on the left instead of the right in the case of a length of one.

Update: In this code format is not print the trailing \s characters. Sample output updated to show the whitespace is missing.

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

for my $val (qw/a ab abc abcd/) {

format =
@||||
$val
.

write \*STDOUT;
}
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Output:

$ ./centre-via-format.pl
  a
 ab
 abc
abcd

$ ./centre-via-format.pl | od -ch
0000000         a  \n       a   b  \n       a   b   c  \n   a   b   c
         2020    0a61    6120    0a62    6120    6362    610a    6362
0000020 d  \n
         0a64
0000022
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