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

print "\nDone. string = '$string'\n";