On the face of it

use strict;
use warnings;

my $str = 'asdfGxxOxxAxxTqwerty';
my $foo = 'GOAT';

my @letters = split //, $foo;
my $match = join '.*', @letters;

$str =~ s/($match)/!$1!/g;

print $str;
[download]

Prints:

asdf!GxxOxxAxxT!qwerty
[download]

does what you want. But I can't help thinking that you are still asking the wrong question. What is the problem you are trying to solve? You have asked how to implement a solution, but we may be able to help more if we know what the actual problem is.