#! perl
use strict;
use warnings;

while (<DATA>)
{
    my @f = split /\s+/;
    my $s = insert_string(@f);

    printf "%9s: %s\n", $f[0], $s;
}

sub insert_string
{
    my ($string, $pos, $insert) = @_;
    my  @chars = split //, $string;

    splice @chars, $pos, 0, $insert;

    return join '', @chars;
}

__DATA__
advark    1 a
aaacccddd 3 bbb
xyz       3 w
qrst      0 p

##</code><code>##

 2:23 >perl 1144_SoPW.pl
   advark: aadvark
aaacccddd: aaabbbcccddd
      xyz: xyzw
     qrst: pqrst

 2:23 >