This is really more of a webserver question.

However, consider that "dirlist.pl" is a perfectly valid directory name.
There might be a /dirlist.pl/docs/index.html and a /dirlist.pl/script/dirlist.pl too.

if you think Perl is involved, you'll have to show some code.

First you have to know that whatever URL you pass to a webserver, it is free to do anything with it. /cgi-bin/dirlist.pl/docs could be a static HTML page.

Usually files below /cgi-bin/ are executed as CGI scripts, and the rest of the URL is passed as some kind of argument to that script. It is free to do with it whatever it likes.

Well ... it could be anything but more than likely it's just path info:

#!/usr/bin/perl

use strict;
use warnings;

use CGI;
use Data::Dumper;

my $cgi = CGI->new();

print $cgi->header,
      $cgi->start_html( 'Path Info?' ),
      $cgi->pre( Dumper( $cgi->path_info() ) ),
      $cgi->end_html;
[download]