@foos = split(/(?=foo)/, 'foo' x 4);
$foos[1] = 'bar';
undef $_;
for $elem(@foos)
{
    $_ .= $elem;
}
print;

##</code><code>##

$_ = 'foo' x 4;
for(map { ++$i == 2 ? 'bar' : $_ } split(/(?=foo/, $_))
{
    $result .= $_;
}
print $result;