use XML::Simple;
my $inst_set = XMLin( 'test.xml', KeepRoot => 1 );

foreach my $Channel (@{$inst_set->{Settings}->{Channels}->{Channel}}) {
  if ( $Channel->{Voltage} ) {
    print "Old voltage: $Channel->{Voltage} -- ";
    $Channel->{Voltage} *= 2.0;
    print "New voltage: $Channel->{Voltage}\n";
  }
}
my $xml = XMLout( $inst_set, KeepRoot => 1 );

print $xml;