There are two things you need your client to tell your server: the file's name and the file's contents. Part of the contents is knowing when the file ends, so either you have to send a length at the beginning, some kind of EOF marker at the end, or else close the connection at EOF.

A simple protocol would be:

> SENDFILE <size> name<eol>
< OK
> (file contents, exactly <size> bytes)
< OK

Assuming you have the file name and size in variables already, this could be as simple as:

print SOCK "SENDFILE $filesize $filename\r\n";
$resp = <SOCK>;
if ($resp !~ /^OK/) { die "Bad response\n" }
while(read(INFILE,$_,8192))
{
  print SOCK;
}
$resp = <SOCK>;
if ($resp !~ /^OK/) { die "Bad response\n" }
[download]