Ok, so here's the dump of relevant packets from Wireshark:

*$EY@gJ!?P"YP\3POST /webappUpload/49930-499301239320229 HTTP/1.1
Cookie: vis=p0BuJAXf0OBO3FMD9obUFbycSoTX3TUD2XOyO7aGCTzzjo5WsNn86yr5Tt
+b10Ylyk6Nu3/ua4bzpmQmspZghGkXehIQnCk0bqQNm0lXV4jN5CyU5lNyltxXJNJjr62l
+0GlVpNf9s2Ic=; __utmx=185998783.00002873392993216020:1:0.000035877605
+79199214:3:1.00003589672086876198:4:1; __utmxx=185998783.000028733929
+93216020:1238293709:2592000.00003587760579199214:1239319412:2592000.0
+0003589672086876198:1239319661:2592000; __utma=185998783.444999910131
+1638000.1238293710.1239163691.1239318674.6; __utmz=185998783.12382937
+10.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __qca=118505064
+2-48687324-6578781; gpermlogin=+Q1/fMvu4oPYsMY2dJwoGMiOqJPYNTxwCJZPMu
+BUCYCz6KI5fsHldCTiIYfqFCXh; __utmv=185998783.Member; _csuid=47196f595
+1870bda; JSESSIONID=EFDC78AE4E5626E4A69BC7D37D7DC767; JSESSIONID=EFDC
+78AE4E5626E4A69BC7D37D7DC767; JSESSIONID=EFDC78AE4E5626E4A69BC7D37D7D
+C767; gathersid=www07; ref=direct_www; __utmb=185998783.10.10.1239318
+674; __utmc=185998783; __qcb=777367231
Referer: http://www.gather.com/shareVideos.action
Cache-Control: no-cache
Connection: Keep-Alive
Accept: */*
Accept-Encoding: text/html
User-Agent: Aurigma Image Uploader JE
Host: media-upload.gather.com
Content-Length: 761879389
Content-Type: multipart/form-data; boundary=XpK3v82IExIWz9o2ChOlFCPaY8
+F5QqDQ3j7y5

--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-d

*$EY@gJ!?P"^jP\0ata; name="memberId"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

49930
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="contentType"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

video
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="albumId"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it


--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="uploadSessionId"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

499301239320229
--

*$EY@gJ!?P"hP\2SourceFileCreatedDateTime_1"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

2009:03:27 04:46:35
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="SourceFileLastModifiedDateTime_1
+"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

2009:03:27 04:46:35
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="SourceFileCreatedDateTimeLocal_1
+"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

2009:03:27 00:46:35
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="SourceFileLastModifiedDateTimeLo
+cal_1"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

2009:03:27 00:46:35
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="FileName_1"
Content-Type: text/plain; charset=UTF-8; Content-Transfer-Encoding: 8b
+it

Exported.avi
--XpK3v82IExIWz9o2ChOlFCPaY8F5QqDQ3j7y5
Content-Disposition: form-data; name="SourceFile_1"; filename="Exporte
+d.avi"
Content-Type: video/x-msvideo; charset=UTF-8; Content-Transfer-Encodin
+g: binary
[download]

Here I'm trying to upload a video called Export.avi. I can definitely see that it's sending something, but not sure how to duplicate this POST request in Perl in order to actually send along the file.