Can you simplify it to this? 
use strict;
use locale;

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

$string =~  s/(_)([^'])/$1'$2/g;

print $string;

__END__

_'nada_'komo_'el_'sol_

[download]

Update: or even: 

$string =~ s/(_)[^']/$1'/g;

[download]

In reply to Re: Modifying the match position after each match by toolic
in thread Modifying the match position after each match by pat_mc

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