$_       = 'foo bbbbbbbah bar';
$find    = 'bah';
$replace = 'ah';

##</code><code>##

$_       = 'foo babab bar';
$find    = 'bab';
$replace = 'xxx';

##</code><code>##

sub replace {
   die "Some helpful messsage" unless 3 == grep defined $_, @_[0..2];
   my ($str,$find,$rep) = @_;
   my $i = 0;
   my $l = length($find);
   my @pos;
   while ($i < length($str)) {
      $i = index($str,$find,$i);
      last if $i < 0;
      push @pos, $i;
      $i += $l;
   }
   substr($str,$_,$l,$rep) for reverse @pos;
   return $str;
}