#! /usr/bin/perl -w

use strict;
use locale;

my $string1 = "_'nada_komo_el_'sol_";
my $string2 = "_na'da_komo_el_so'l_";

$string1 =~  s/(_)([^']+?)(_)(?{pos($string1) --})/$1'$2$3/go;
$string2 =~  s/(_)([^']+?)(_)(?{pos($string2) --})/$1'$2$3/go;

print "String 1: $string1 \n";
print "String 2: $string2 \n";

##</code><code>##

--output:--
String 1: _'nada_'komo_el_'sol_
String 2: _na'da_'komo_el_so'l_