my $str = "blublublublublu";
my $such='lu';
my $repl='LA';

my $count = 3;
my $pos=0;

while($count and $pos >= 0)
{
  $pos=index($str,$such,$pos);
  $count--;
}

if($pos>0){
  substr($str,$pos,length($such),$repl);
}