Re^2: Elegant way to split into sequences of identical chars?

[xdg]

That's really cool, but I'm confused as to why it works. From perlre about (??{ code }):

This is a "postponed" regular subexpression. The "code" is evaluated at run time, at the moment this subexpression may match. The result of evaluation is considered as a regular expression and matched as if it were inserted instead of this construct.

Maybe I'm thrown by the "matched as if it were inserted" part when what's being inserted is a regular expression -- if it were just inserted, I don't understand why your approach works different from just putting the subexpression in directly. It looks like it's being evaluated somehow "separately" from the rest of the regular expression, which I didn't expect given the doc. I don't know if that makes sense, but this code sample illustrates where I'm thrown:

my $re = qr/(.)\1*/;

@matches = $string =~ m/($re)/g;
print "@matches\n"; # x x x x 5 5 5 5 6 6 x x x x

@matches = $string =~ m/((??{$re}))/g;
print "@matches\n"; # xx 55 6 xx

Could you please help me understand what's really going on that's different between these two cases?