the tags have to be zero characters long for the wrapping purposes.

That's more difficult. I would probably do it like this:

$_ = "foo<span>bar</span>bazqux<span>quux</span>";

my $c=0;
s( (<span>|</span>|.) ) {
    my $s = $1;
    $s =~ m!</?span>! ? $s : $s . (++$c % 4 ? '' : "\n");
}sxge;

print;
print "\n";
[download]

That will show you how it works but in your case you'll need to change that '4' to '80' in the ++$c % 4 part.

Update: added /s to regex so . would match "\n" as well.

-sauoq
"My two cents aren't worth a dime.";

Not bad, but why try to match the tag again when you already let the engine match it before?

$_ = "foo<span>bar</span>bazqux<span>quux</span>";

my $c=0;
s( (<span>|</span>|(.)) ) {
    defined $2 ? $1 . (++$c % 4 ? '' : "\n") : $1;
}sxge;

print;
print "\n";
[download]

Makeshifts last the longest.

Assuming you never have an empty <span></span> set, I believe the following should work, should it not?

s{((?:(?:<span>)?.(?:</span>)?){80})}{$1\n}g

bbfu
Black flowers blossum
Fearless on my breath

Assuming you never have an empty <span></span> set, I believe the following should work, should it not?

s{((?:(?:<span>)?.(?:</span>)?){80})}{$1\n}g
[download]

No.

For illustrative purposes, assume that you want to match 10 characters rather than 80. That won't change the behavior. Let's try it on something simple like "<span>1</span>":

 
#!perl 
$_ = "<span>1</span>";
s{((?:(?:<span>)?.(?:</span>)?){10})}{$1\n}g;
print "$_\n";
__END__
<span>1</s
pan>
[download]

What happened? Well, perl tries hard to make the expression match. When it doesn't succeed at first by matching the initial span tag with the first optional group, it backtracks and matches that first "<" with the dot. This isn't restricted to one or two edge cases either. It may work for "<span>foo</span>" but only because "foo</span>" is exactly 10 characters. It won't work for "<span>fo</span>" or "<span>foob</span>". Of course, the examples would be different if you are trying to count 80 characters but the bug is the same.

-sauoq
"My two cents aren't worth a dime.";