use strict;
use warnings;
use 5.010;

my $string = "_'nada_komo_el_'sol_";

$string =~  s/_'?(.)/_'$1/g;

say $string;

--output:--
_'nada_'komo_'el_'sol_

##</code><code>##

use strict;
use warnings;
use 5.010;


my $string = "_'nada_komo_el_'sol_";

my @pieces = split /_'?/, $string;

say join(q{_'}, @pieces);

--output:--
_'nada_'komo_'el_'sol