Using XML::LibXML,

use strict;
use warnings;
use XML::LibXML qw( );

my $file_name = '...';

my $parser = XML::LibXML->new();
my $doc = $parser->parse_file(...);
my $root = $doc->documentElement();

for my $node ($root->findnodes(
   '//Parameter[@Name="MapSource"]'
   .'/Value/Item[@Value="NONE"]'
)) {
   $node->setAttribute(Value => 'API');
}

open(my $fh, '>:bytes', $file_name) or die;
print($fh $doc->toString());
[download]

Update: Forgot to select correct Parameter. Fixed.