Here's an (almost) one-line solution.

It took a few minutes, but this looks like a good prototype for your needs. You will likely need to adjust the tests on the position variable $i to fit your need more precisely...

use strict;
my $str = 'a,b,c,d,e,f,g,h,i,j,k,l,m,n';
print "$str\n";

my $i = 0;
$str =~ s/(.)/$i++;($1 ne ',' or $i>15 && $i<25) ? $1 : '#'/ge;

print "$str\n";
[download]

This prints:

a,b,c,d,e,f,g,h,i,j,k,l,m,n
a#b#c#d#e#f#g#h,i,j,k,l,m#n
[download]

I used '#' instead of "\t" for demo purposes so things would line up nicely in the print test.

Note: along the way I tried using pos instead of $i, but apparently it is undefined while still in the right side of the s///. Update: Hofmator is correct to say I am wrong to imply that pos would work outside the s///.   pos is not set at all for s///.

Note 2: I offer this as a solution to the problem as you described it but it seems very odd that you would have a list of companies with names that are exactly 10 characters long.