$ ls -lR
.:
total 4
drwxr-xr-x 1 ken None   0 Oct  1 05:18 u1
drwxr-xr-x 1 ken None   0 Oct  1 04:00 u2
drwxr-xr-x 1 ken None   0 Oct  1 04:33 u3
-rwxr-xr-x 1 ken None 839 Oct  1 06:07 web_dir_listing.pl

./u1:
total 5
-rw-r--r-- 1 ken None 2002 Oct  1 05:11 u1_a
-rw-r--r-- 1 ken None  501 Oct  1 05:18 u1_b

./u2:
total 4
-rw-r--r-- 1 ken None 4004 Oct  1 05:18 u2_c
-rw-r--r-- 1 ken None    0 Oct  1 04:00 u2_d

./u3:
total 0

$ ./web_dir_listing.pl u1
<p>User: <strong>u1</strong></p>
<ul>
<li>-rw-r--r-- 1 ken None 2.0K Oct  1 05:11 <a href="https://www.example.com/reg/u1/u1_a">u1_a</a></li>
<li>-rw-r--r-- 1 ken None  501 Oct  1 05:18 <a href="https://www.example.com/reg/u1/u1_b">u1_b</a></li>
</ul>

$ ./web_dir_listing.pl u2
<p>User: <strong>u2</strong></p>
<ul>
<li>-rw-r--r-- 1 ken None 4.0K Oct  1 05:18 <a href="https://www.example.com/reg/u2/u2_c">u2_c</a></li>
<li>-rw-r--r-- 1 ken None    0 Oct  1 04:00 <a href="https://www.example.com/reg/u2/u2_d">u2_d</a></li>
</ul>

$ ./web_dir_listing.pl u3
<p>User: <strong>u3</strong></p>
<p>User 'u3' has no files.</p>

$ ./web_dir_listing.pl u4
ERROR: User 'u4' does not exist.

$ ./web_dir_listing.pl 'u1;pwd'
ERROR: User 'u1;pwd' does not exist.

$ ./web_dir_listing.pl
Usage: ./web_dir_listing.pl dir