my $string = "abc abc abc abc";
my $x = 1;
# while ( $string =~ /abc /ig ) {
while ( $string =~ /abc/ig ) {
    my $i = pos($string);
#    substr( $string, $-[0], $+[0]-$-[0]) =~ s/(abc) /$1 def($x)/;
    substr( $string, $-[0], $+[0]-$-[0]) =~ s/(abc)/"$1 def(" . $x++ .")"/e;
    print "$string\n";
    pos($string) = $i + 6;
}