package Mojolicious::Plugin::MySub;
use Mojo::Base 'Mojolicious::Plugin';

sub register {
   my $self = shift;
   my $app  = shift;

   $app->helper( mysub => sub { 1 } );
}

1;

## app.pl

#!/usr/bin/env perl

use Mojolicious::Lite;
plugin 'Mojolicious::Plugin::MySub';

mysub '/test' => sub {
  my $self = shift;
};
app->start;