#!/usr/bin/perl -wT
use strict;

my $string  = 'abc ' x 10;
my $search  = 'ab';
my $replace = 'CD';

print "$string\n";

my $i = 0;
while ($string =~ /$search/gi) {
  substr($string, $-[0], $+[0]-$-[0], $replace);
  last if ++$i == 4;
  pos($string) = $-[0] + 1;
}

print "$string\n";

__END__
abc abc abc abc abc abc abc abc abc abc
CDc CDc CDc CDc abc abc abc abc abc abc