You can do this:
<%= select(:preferences, :job_history_filter_period, ContactPreference::JOB_HISTORY_OPTIONS, options = {}, html_options = { :onChange => "new Ajax.Request('/controller_name/modify_history_time_period/' + this[this.selectedIndex].value, {asynchronous:true, evalScripts:true});" } ) %>
It's not as clean as the helper but gets the job done.
Cheers, Atha