sub
sub - declare a subroutine, possibly anonymously
sub BLOCK
sub NAME
sub NAME BLOCK
This is subroutine definition, not a real function per se. With just a
NAME (and possibly prototypes), it's just a forward declaration. Without
a NAME, it's an anonymous function declaration, and does actually return a
value: the CODE ref of the closure you just created. See
the perlsub manpage
and
the perlref manpage
for details.