Please edit your post to correct code-tags. Additionally 'hello world add' doesn't make sense.

Please see How do I post a question effectively?

The better the question the better the answers!

I need to guess what you really want:

my $input    = "good morning all";
my $pattern  = qr/good morning (\w+)/;
my $replace  = '"hello $1"';

my $output = $input =~ s/$pattern/$replace/ree;

print $output;
[download]

hello all
[download]

Please be sure to have full control of the $replace strings since the eval /ee would lead to security issues.