You have to use

use locale;
use POSIX 'locale_h';

my $locale        = 'Bulgarian_Bulgaria.1251';
# get the current locale
my $org_locale    = setlocale(LC_CTYPE);
# set the current locale
setlocale(LC_CTYPE, $locale) or die "Invalid locale $locale";
# do some work like 
$words = ucfirst$word;
# set again the original locale 
setlocale(LC_CTYPE, $org_locale) or die "Invalid locale $org_locale";
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