May be it is something from outside perl. Have you configured your locale variables in your system. Try set and see your locales LC_ALL, etc...

It happened to me (also Spanish) and it was a mess to distinguish if the problems was in perl or from my local environment variables. Please, check all your default variables before trying with perl's.

use POSIX qw(strftime setlocale LC_ALL LC_CTYPE); 
my ($loc) = POSIX::setlocale( &POSIX::LC_ALL, 'es_ES.ISO8859-1' ); 
my ($now_string) = strftime "%a %b %e %H:%M:%S %Y", localtime; 
my ($fecBita)    = strftime "%Y-%m-%d %H:%M:%S",    localtime;
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Your code is Ok!