Basically an anonymous sub is just a sub that doesn't have a name in the package, so you just hold a reference to it, like when you make \&foo to make a reference to a sub foo {}.

But the most usefult thing of anonymous sub is closure:

sub foo {
  my ( $arg ) = @_ ;
  
  my $bar = sub {
    print "ARG[$arg]\n" ;
    return "RE[$arg]" ;
  } ;
  
  return $bar ;
}

  my $bar1 = foo(123) ;
  my $bar2 = foo(456) ;
  
  my $foo1 = &$bar1() ;
  my $foo2 = &$bar2() ;
  
  print "foo1: $foo1\n" ;
  print "foo2: $foo2\n" ;
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Output:

ARG[123]
ARG[456]
foo1: RE[123]
foo2: RE[456]
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Also is useful to create functions with eval:

  my $e = 10 ;
  my $sub = eval(" sub { return 2**$e } ") ;
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You also can redefine a normal sub with the declaration of a anonymous sub:

  sub foo { print "foo\n" ;}
  # now redefining it:
  *foo = sub { print "bar\n" ;}
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See perlref and perlsub