Would this work?

#!/usr/bin/perl

use strict;

my @file = <DATA>;
foreach my $i (@file) {
    $i =~ s/\n/ /;
    $i =~ /^A\d\.\d\./ ? print "\n$i" : print $i;
}

__DATA__
A6.8.5 This is
an example
sentence
A6.8.6 This is
an example
sentence
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