どうして[aa][aa][aa]や[aa]b[aa]ab[aa]にはならないのですか
https://ideone.com/WKK9DO

my $str = 'abab';
$str =~ s/(?<=\Ka)b/[$&]/g;
print ${str}; # [ab][ab]

my $str = 'aaaaaa';
$str =~ s/(?<=\K.)./[$&]/g;
print $str; # [aa]aaaa

my $str = 'aabaaabaa';
$str =~ s/(?<=\Ka)a/[$&]/g;
print $str; # [aa]b[aa]abaa