$ 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.
$

##</code><code>##

$ perl -le '
> $str = q{adfsdheHjdGafefaffJaff};
> $str =~ s{(?:(?<=^)|(?<=[A-Z]))a}{999}g;
> print $str;'
999dfsdheHjdG999fefaffJ999ff
$