Hello tkguifan,

For your particular problem, substr is the best solution, as others have shown. However, you might find it useful to see how your original approach can be made to work. The missing piece of the puzzle is splice:

#! 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
[download]

Output:

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

 2:23 >
[download]

Hope that helps,