Doh! I didn't see your very similar solution before I posted....

It is very similar, isn't it? And I hadn't seen yaphy's solution before I posted mine. Now, it left me wondering: is setting pos() necessary, or will Perl DWIM, and continue matching at the same point in the (original) string without help? That's what s///g does.

Well, it turns out that it appears to be using an offset in the string to keep track of where it was. So it is necessary.

$_ = 'abxy' x 12;
for(my $i = 0; m/ab/g and $i++<5;) {
    substr($_, $-[0], $+[0]-$-[0]) = 'abXab';
}
print;
[download]

Result: abXabXabXabXabXabxyabxyabxyabxyabxyabxyabxyabxyabxyabxyabxyabxy

Not good.

$_ = 'abxy' x 12;
for(my $i = 0; m/ab/g and $i++<5;) {
    substr($_, $-[0], $+[0]-$-[0]) = 'abXab';
    pos = $-[0]+5;
}
print;
[download]

Result: abXabxyabXabxyabXabxyabXabxyabXabxyabxyabxyabxyabxyabxyabxyabxy

Good.

p.s. I noticed this, which is also very awkward:

my $length = length(substr($_, 2, 2) = 'abXab');
print $length;
[download]

prints 2, not 5. What happened to the rule: the value of an assignment as an expression, is what you assign?