The difference you point out is even more blatant in this example:
$\ = "\n";
my $s = 'abcdefghij';
$s =~ s/def/123/;
print $&; # def
print substr($s, $-[0], $+[0] - $-[0]); # 123
Some code is surely using that.
The problem with $& and friends isn't the location of the partial strings but the semantics that require copying the parts. Thus for every match, the entire string must be copied.
Anecdote:
A poster on clpmisc complained that his program "stopped working" after he introduced use English. He was matching a short pattern against some monster genome string (one or two GB) with a reasonable number of matches (some 10_000). Copying killed the cat, to coin a phrase.
Anno
