You have too old a perl, try

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1220510

use strict;
use warnings;

while( <DATA> )
  {
  s/(^|    )(?!\d)/$&0/g;
  print;
  }

__DATA__
1    2            2        5                4
4    4    4            4                3    
        4    4            1                
    1    5    6        4
[download]