my $a = "17341234";

##</code><code>##

$a=~s/(.)(...)\1/A$2B/g;
print "$a\n";

##</code><code>##

A7AAB2BB

##</code><code>##

A734B234

##</code><code>##

$a=~s/(.)(?=...\1)/A/g;
$a=~s/(?<=A...)(.)/B/g;
print "$a\n";

##</code><code>##

A7AAB2BB