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#!/usr/bin/perl -wl
use strict;
...
    *_ = \substr $_, pos()-4 while s/(.)(...)\1/A$2B/;
}
print $str;

    my $i = 0;
    *_ = \substr $str, $i while s/(.)(...)\1(?{ $i += pos() - 4 })/A$2
+B/;