gistfile1.txt

package Plack::Builder;

sub new { ... }

sub enable {
    my $self = shift;
    ...
}

sub enable_if {
    my $self = shift;
    ...
}

sub mount {
    my $self = shift;
    ...
}

package Plack::Builder::DSL;
our @EXPORT = qw( builder enable enable_if mount );

my $builder = Plack::Builder::Unbound->new;

sub builder {
    my $block = shift;

    (my $old, $builder) = ($builder, Plack::Builder->new);
    my $app = $block->();
    my $res = $builder->to_app($app);
    $builder = $old;
    
    $res;
}

sub enable    { $builder->enable(@_) }
sub enable_if { $builder->enable_if(@_) }
sub mount     { $builder->mount(@_) }

package Plack::Builder::Unbound;

sub new { ... }

sub enable { croak "Can't call enable() outside builder {} block" }