G'day liuweichuan,

"which modules or functions for comparing text similarity by perl"

That's extremely vague! What sort of text? What format? What type of comparison? What else can you tell us that will help us to help you?

There are operators, such as cmp: see perlop. There are regular expressions: see perlre. There are all sorts of string-related functions: see Perl functions A-Z. There are any number of builtin modules that might be useful: see the "Modules" section of http://perldoc.perl.org/perl.html.

Then there's CPAN. Try searching: http://search.cpan.org/. I found these fairly quickly: Text::Compare; Text::Similarity; Text::Soundex.