Did you actually examine the POST command you have on your UNIX system? ;)

$ head /usr/local/bin/POST
#!/usr/local/bin/perl -w

eval 'exec /usr/local/bin/perl -w -S $0 ${1+"$@"}'
    if 0; # not running under some shell

# $Id: lwp-request,v 2.6 2003/10/26 14:39:18 gisle Exp $
#
# Simple user agent using LWP library.

=head1 NAME
[download]

So yeah, it's just a simple Perl script that uses the LWP module. See perldoc /usr/local/bin/POST for the POD on that (please note: on my system, POST is located in /usr/local/bin. On your system it could be different, but a simple which POST will show you the location).