pkupnorth, you changed 3 things:

• Changed from 'GET' to GET => $endpoint
• Moved the 'Accept' to a separate $req->header() line
• Changed from 'Accept' => ... to Accept => ...

If you do them one at a time, you will find that it was moving the Accept from the ->new(...) to a separate $req->header() line that got rid of the "Bad header argument at code.pl line 8." error, not the removal of the single quotes.

 use strict;
 use warnings;
 use LWP::UserAgent;
 use HTTP::Request::Common qw(GET);

 my $ua = LWP::UserAgent->new;
 my $endpoint = 'Http://www.sierraaluminum.com:8081/api/MarvinWindows/
+getASN';
 my $req = HTTP::Request->new('GET');
 $req->header('Accept' => 'application/xml');
 $req->url($endpoint);
 my $resp = $ua->request($req);
 my $message = $resp->content;
 print "printing message: $message\n\n";
[download]

update: fixed the order of the quoted version in the bullet list

update 2: added "not the removal of the single quotes"; removed an unnecessary sentence that was unnecessarily argumentative in tone.