No real idea here, but depending on where you are testing from, it could be an instance of the old 'broken Path-MTU-discovery.'

Basically, if there is a (broken) firewall between your client and the server which is dropping all ICMP packets, one symptom you can see is small transfers working fine but larger transfers failing. (The client TCP tries to determine the optimum packet size by increasing the size until it receives an 'I had to fragment your packet' ICMP message...if this doesn't get back to the sender the data gets discarded :-(

You can try searching networking archives and such or your system documentation. If you can disable path-mtu-discovery on the client and that helps then go and beat up a firewall admin.

Other than that, does the script work fine if you run from the command line? i.e. does perl my_script.pl < form_parameters work OK?

If not, and you can reproduce the problem this way, then all I can suggest is 'perl -d'.

Let us know how you get on.