my $translation_table = {
  'A' => 'A',     # translate from => to
  'a' => 'a',
  'r' => 'r',
  # etc
};

my $str = "Barra O Gri obhtha";

# build search pattern as 'pattern1|pattern2|...'

my $patterns = join '|', keys %$translation_table;

$str =~ s/($patterns)/$translation_table->{$1}/ge;