The POST data formatting for www/x-www-form-urlencoded requests is just the same as the formatting for GET, only the data goes in the request body instead of at the end of the URL. You could use URI::Escape

use URI::Escape;
my $postbody;
while (my ($key,$value) = each %params) {
   $postbody .= "&" if defined $postbody;
   $postbody .= uri_escape($key)."=".uri_escape($value);
}
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