Shouldn't that be "window.location.href"?
Anyway, using a browser that actually gives decent Javascript error messages (Mozilla) points out that there's a semicolon missing after the alert() call.
With those two changes the code becomes:
echo "<script Language=\"JavaScript\" Type=\"text/javascript\">\n";
echo "<!--\n";
echo "alert('Updated');";
echo "window.location.href = 'http://www.123456.com/per_mod.php?op=curmod';";
echo "//-->\n";
echo "</script>\n";
WFM.