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A subroutine may be called using an explicit & prefix. The & is optional in modern Perl, as are parentheses if the subroutine has been predeclared. The & is not optional when just naming the subroutine, such as when it's used as an argument to defined() or undef(). Nor is it optional when you want to do an indirect subroutine call with a subroutine name or reference using the &$subref() or &{$subref}() constructs, although the $subref->() notation solves that problem. See the perlref manpage for more about all that.
Not only does the & form make the argument list optional, it also disables any prototype checking on arguments you do provide. This is partly for historical reasons, and partly for having a convenient way to cheat if you know what you're doing. See Prototypes below.
Beides aus perldoc perlsub.
Alexander
Nein, ich beantworte keine Fragen per eMail. Dafür ist das Forum da.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so!"