The -s switch is a little problematic on Win32, at least with AS 5.6. perl -s script -VAR=1 works fine but without the -s on the command line (ie with #!perl -s) it does not work. In other words it seems to me that while -w is read off the shebang on Win32 -s is not. Anyway -s implements a very simple mechanism. I would suggest using one of the Getopt modules is a better way to go but you can fake its behaviour with a handful of lines:

C:\>type test.pl
#!/usr/bin/perl

use strict;
our ( $HELLO, $WORLD );

# fake the behaviour of -s
{
    no strict;
    my @other = ();
    while(my $term = shift @ARGV) {
        $term =~ s/^\-/\$/ ? eval $term : push @other, $term;
    }
    @ARGV = @other;
}

print "Got $HELLO $WORLD\n";
print "\@ARGV = ( @ARGV )\n";

C:\>perl test.pl -HELLO=hello -WORLD=world other arguments
Got hello world
@ARGV = ( other arguments )
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Note that this code does a trusting string eval which is dangerous but on Win32 your security is already shot ;-). You should probably make it more secure by validating the $term string before the eval but as whoever executes it already has a shell in which to execute arbitrary commands (and the script will exec with their perms) I don't really see it as a major security issue.