#!/usr/bin/perl
use warnings;
use 5.012;
use utf8;
use open qw( :encoding(UTF-8) :std );
use Mojolicious::Lite;
use File::Spec::Functions;

my $dir = '/tmp';
my $file_1 = catfile $dir, 'file_one.txt';
my $file_2 = catfile $dir, 'file_two.txt';


get '/eins' => sub {
    ...
    open my $fh, '<', $file_1 or die $!;
    ...
}

post '/zwei' => sub {
    ...
    open my $fh, '>', $file_1 or die $!;
    ...
}