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my $foo = sub { print "Foo\n"; };
my $bar = sub { print "Bar\n"; };
...
$x->();  # prints "Foo"
$x = $bar;
$x->();  # prints "Bar"

my %code = (
    foo => sub { print "Foo\n"; },
...
);
$code{foo}();  # prints "Foo"
$code{bar}();  # prints "Bar"

sub bar {
    my( $output ) = @_;
...
my $foo2 = sub { print "Bar\n"; };
bar( $foo1 );  # prints "Foo"
bar( $foo2 );  # prints "Bar"