Question: how can I invoke Perl from the command-line so that these file-handles are properly initialized?

I'd not be very surprised if there was a module on CPAN that would do this for you, but I never searched for one.

See RFC 3875 or earlier.

Here's code for a simple, single-threaded HTTP server which you can use to debug your perl CGI scripts. All requests will be directed to your code. I've tested it with file uploads, and it seems to work just fine.

You can also use it to save requests to a file which can be useful for generating the input files required to test your CGI scripts off-line. See the comment near the end of the main loop.

To use, insert your perl code in the call_cgi routine. It will listen on port 9000 (which can be changed by supplying a port number on the command line.) You'll also have change the ACTION parameter of your FORM to direct requests to this server (e.g. ACTION="http://localhost:9000/".)

#!/usr/bin/perl
#
# quick and dirty signle threaded CGI executive
# can also be used to capture requests

use strict;
use warnings;
use IO::Socket::INET;
use IO::String;

my $port = shift(@ARGV) || 9000;
my $listen = IO::Socket::INET->new(
    Listen    => 5,
    LocalAddr => 'localhost',
    LocalPort => $port,
    Proto     => 'tcp',
    ReuseAddr => 1
);

unless ($listen) {
  die "unable to listen on port $port: $!\n"
};

while (1) {
  print STDERR "waiting for connection on port $port\n";
  my $s = $listen->accept();

  open(STDOUT, ">&=".fileno($s));
  open(STDIN, "<&=".fileno($s));

  my ($req, $content);
  delete $ENV{CONTENT_LENGTH};
  { local ($/) = "\r\n";
    while (<STDIN>) {
      $req .= $_;
      chomp;
      # print STDERR "got: $_\n";
      last unless /\S/;
      if (/^GET\s*(\S+)/) {
        $ENV{REQUEST_METHOD} = 'GET';
        (my $qs = $1) =~ m/\?(.*)/;
        $ENV{'QUERY_STRING'} = $1;
      } elsif (/^POST/) {
        $ENV{REQUEST_METHOD} = 'POST';
        $ENV{'QUERY_STRING'} = '';
      } elsif (/^Content-Type:\s*(.*)/) {
        $ENV{CONTENT_TYPE} = $1;
      } elsif (/^Content-Length:\s*(.*)/) {
        $ENV{CONTENT_LENGTH} = $1;
      }
    }
  }
  if (my $size = $ENV{CONTENT_LENGTH}) {
    $content = '';
    while (length($content) < $size) {
        my $nr = read(STDIN, $content, $size-length($content),
                      length($content));
        die "read error" unless $nr;
    }
  }

  # can save $req, $content here:
  # open(F, ">request"); print F $req, $content; close(F);

  close(STDIN); # n.b.: does not close socket
  tie *STDIN, 'IO::String', $content;

  undef @CGI::QUERY_PARAM;
  call_cgi();

  untie *STDIN;
  close(STDOUT);
  close($s);
}

sub call_cgi
{
    print "HTTP/1.0 200\r\n";

    # your CGI code goes here, example code follows

    use CGI;
    my $q = new CGI();

    print $q->header(-type => 'text/plain');
    print "Color = ", $q->param('color'), "\n";
    my $file = $q->param('myfile');
    print "myfile = ", $file, "\n";

    print "myfile contents:\n";
    while (<$file>) { print; }
    print "end of file\n";
}
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