/A/g;
s/\G(.*?)A/$1a/g;

##</code><code>##

s{(?<=A)(.*)}{my $rest=$1; $rest=~tr/A/a/; $rest}e;

##</code><code>##

substr($_, index($_, 'A')+1) =~ tr/A/a/;

##</code><code>##

/A/g;
substr($_, ++pos) =~ tr/A/a/;