Thanks you :-) After googling for a while, I find http://coding.derkeiler.com/Archive/Perl/comp.lang.perl.misc/2005-07/msg00757.html helpful, I now using the following scheme to set multiple value for a <select> input, and it works well. 
$mech->select("options", ['1', '3']);

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In reply to Re^2: How to handle multiple select in WWW::Mechanize? by phio
in thread How to handle multiple select in WWW::Mechanize? by phio

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