perldoc -f my
my EXPR
my TYPE EXPR
my EXPR : ATTRS
my TYPE EXPR : ATTRS
        A "my" declares the listed variables to be local (lexically) to
        the enclosing block, file, or "eval". If more than one value is
        listed, the list must be placed in parentheses.

        The exact semantics and interface of TYPE and ATTRS are still
        evolving. TYPE is currently bound to the use of "fields" pragma,
        and attributes are handled using the "attributes" pragma, or
        starting from Perl 5.8.0 also via the "Attribute::Handlers"
        module. See "Private Variables via my()" in perlsub for details,
        and fields, attributes, and Attribute::Handlers.