Before using location_constraint you must pass the host parameter to the Net::Amazon::S3 constructor. (Note I have never found it necessary to use the param after connecting in this way).

Here's how I do it:

$conf = {
    aws_access_key_id     => 'foo',
    aws_secret_access_key => 'bar',
    host                  => 's3-us-west-1.amazonaws.com',
};

my $s3_base = Net::Amazon::S3->new( $conf );
my $s3      = Net::Amazon::S3::Client->new( s3 => $s3_base );

my $bucket = $s3->bucket( name => $bucket_name );

...
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Hope this helps!