I wouldn't know how to make it stop redirecting in the same window, but if the data is lost in the new window maybe you could try a url_param. Instead of post.pl, use something like

my $forminfo = param('forminfo');
post.pl?stuff=$forminfo
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Then in post.pl, do something like:

my $forminfo = param('stuff');

if ($forminfo)
{
 print "Hello world!\n";
 print "$forminfo";
}
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I have no idea if this will help your problem, but if you pass the paramaters as part of the URL itself, maybe it will help.