my %hash = ( test => 'ÄÖÜß' );
my $encoding = 'utf8';

use JSON;

package JSON;
sub set_encoding {
  my $self = shift;
  my $encoding = shift;
  return eval "\$self->can($encoding) && \$self->$encoding";
}
1;

my $json = JSON->new;

$json->set_encoding($encoding) or warn "unknown encoding: $encoding";
print $json->encode(\%hash);
     
print "\n";

$encoding = 'latin1';
$json->set_encoding($encoding) or warn "unknown encoding: $encoding";
print $json->encode(\%hash);

print "\n";
$encoding = 'deutsch1968';
$json->set_encoding($encoding) or warn "unknown encoding: $encoding";
print $json->encode(\%hash);