nginx.conf

    server {
        listen 80; 
        server_name example.com;
        proxy_cache one;
        proxy_cache_lock on; 
        proxy_cache_valid 1m; 
        location /a/ {
            proxy_pass http://127.0.0.1:5000;
        }   
        location /b/ {
            proxy_pass http://127.0.0.1:5000;
        }   
    }   

xxx.psgi

use v5.14;
use strict;
use warnings;
use Time::HiRes qw(time);

return sub {
    my $env = shift;
    if ( $env->{REQUEST_URI} =~ /b/ ) {
        use Data::Dumper;
        print STDERR Dumper($env);
        say readline $env->{'psgi.input'};
        return [200, ['content-type' => 'text/plain'], ['hello '. time]];
    } else {
        return [200, ['content-type' => 'text/plain', 'X-Accel-Redirect' => '/b/'], ['']];
    }  
};