$ perl -le '
> $str = q{adfsdheHjdGafefaffJaff};
> $str =~ s{(?<=^|[A-Z])a}{999}g;
> print $str;'
Variable length lookbehind not implemented in regex; marked by <-- HERE in m/(?<=^|[A-Z])a <-- HERE / at -e line 3.
$