my $string = "abc abc abc abc";
my $x = 1;
while ($string =~ /abc /ig) {
    my $i = pos($string);
    $string =~ s/(abc) /$1 def($x)/;
    print "$string\n";
    pos($string) = $i + 6;
}
print "\nDone. string = '$string'\n";


##</code><code>##

abc def(1) abc def(2) abc def(3) abc def(4)