In what sense does your snippet as shown not work?

Can you wrap the snippet in a small (20 lines) self-contained program that shows the problem?

Note that the call to External::Parser->run will always have happened before your function returns. This is different from the example in Promises because the http_get function is an asynchronous function which invokes the callback upon completion. If you want to move some arbitrary synchonous code to Promises, I would try to wrap a callback to that code with an object that implements a function ->result that actually calls the synchronous code and returns the result:

package Promise::Lazy;
use parent 'Promises::Promise';

sub new {
    my( $class, $usercallback ) = @_;
    my $self = $class::SUPER->new();
    $self->{ usercallback } = $usercallback;
    $self
};

sub result {
    my( $self ) = @_;
    my $result = $self->{ usercallback }->();
    if( $result ) {
        $self->resolve( $result )
    } else {
        $self->reject();
    };
};

... somewhere in your code ...

return Promise::Lazy->new(sub {
    External::Parser->run('--in', $file);
});
[download]