Win8 Strawberry 5.30.3.1 (64)  Sat 12/04/2021 21:26:27
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
my $str='kklmn';

#        +--------------- if you look ahead to kkl
#        |
#        |  +---+-------- while also capturing it to capture group 1
#        |  |   |         (match point is not advanced)
#        |  |   |
#        |  |   |    +-------- and then if capture 1 captured anything
#        |  |   |    |
#        |  |   |    |  +----- match with what was captured to $1
#        |  |   |    |  |
#        |  |   |    |  |  +-- else match with mn
#        |  |   |    |  |  |
#        |  |   |    |  |  |    +------- and substitute this
#        |  |   |    |  |  |    |
#        v  v---v    v  v  v    v
$str=~ s/(?=(kkl))(?(1)\g1|mn)/***/g;

print "'$str' \n";
^Z
'***mn'