bless tells Perl to mark the item referred to by REF as an object in a package. The two-argument version of bless is always preferable unless there is a specific reason to not use it.
Bless the referred-to item into a specific package (recommended form):
bless $ref, $package;The two-argument form adds the object to the package specified as the second argument.
Bless the referred-to item into package main:
bless $ref, "";If the second argument is an empty string, bless adds the object to package main.
Bless the referred-to item into the current package (not inheritable):
bless $ref;If bless is used without its second argument, the object is created in the current package. The second argument should always be supplied if a derived class might inherit a method executing bless. Because it is a potential source of bugs, one-argument bless is discouraged.
See perlobj for more about the blessing (and blessings) of objects.
bless returns its first argument, the supplied reference, as the value of the function; since bless is commonly the last thing executed in constructors, this means that the reference to the object is returned as the constructor's value and allows the caller to immediately use this returned object in method calls.
CLASSNAME should always be a mixed-case name, as all-uppercase and all-lowercase names are meant to be used only for Perl builtin types and pragmas, respectively. Avoid creating all-uppercase or all-lowercase package names to prevent confusion.
Also avoid blessing things into the class name 0; this will cause code which (erroneously) checks the result of ref to see if a reference is blessed to fail, as "0", a falsy value, is returned.
See "Perl Modules" in perlmod for more details.