Update: Enlil points out that this suffers from the same problem as the OP's own solution, highlighted by BrowserUk.

#!/usr/bin/perl -wl
use strict;
$_="17341234";
s/(.)(?=...\1)|(?<=(.)...)\2/defined $1 ? "A" : "B"/eg;
print;
__END__
A7AAB2BB
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Makeshifts last the longest.