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);
});