my $query_string;

foreach (keys %nvpairs) {
 # guess which module defines uri_escape =)
 $query_string .= "&" if $query_string;
 $query_string .= "$_=". uri_escape($nvpairs{$_});
}

# assuming $q is a CGI query object ...

print $q->a({href=>"http://www.foo.com?$query_string"}, "click me!");