As Joost suggested, you may have to build it from other environment vars. To get a list of all the environment vars your install of IIS provides to perl, you could dump the %ENV hash to a file like so:

#!c:/perl/bin/perl -w

use strict;
use Data::Dumper;

open my $ENV_DUMP, '>', 'c:/iis_env.txt' or die $!;
print {$ENV_DUMP} Dumper(\%ENV);
close $ENV_DUMP;

print "Content-Type: text/plain\n\nSuccess!";
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Of course, you want to run this through IIS, not the Windows command line.