I try to read the code in FormHandler but still cannot find the way to fill default value into the input text.

Try reading the docs, and the examples (test suite) instead

http://search.cpan.org/~gshank/HTML-FormHandler-0.36002/lib/HTML/FormHandler.pm#defaults

http://search.cpan.org/grep?cpanid=GSHANK&release=HTML-FormHandler-0.36002&string=default&i=1&n=1&C=0

http://search.cpan.org/~gshank/HTML-FormHandler-0.36002/MANIFEST

http://search.cpan.org/src/GSHANK/HTML-FormHandler-0.36002/t/defaults.t