$ perl -le '
> $str = qq{Keyword string\nBlah\nKeyword\nstring\nBlech\nKeyword "string"};
> print $str;
> print;
> $str =~ s{Keyword(\s+"?)string}{Keyword${1}1_string}sg;
> print $str;'
Keyword string
Blah
Keyword
string
Blech
Keyword "string"

Keyword 1_string
Blah
Keyword
1_string
Blech
Keyword "1_string"
$