OK, Two methods:

1. Use the Perl Data Base Interface (DBI) module
   This is a Perl module that lets you write database code in standard SQL regardlesss of the actual database you are using, including flat-files. You'll pretend you're using an actual rdbms, write your code in sql, and DBI will take care of translating that into the commands that will interface with your flat-file. A good introduction to the Perl DBI is http://www.perl.com/pub/a/1999/10/DBI.html. Then, if you decide later on to switch to a real db, you'll have just a few lines of your code to change as you tell DBI which type of db to interface with, instead of the whole thing to re-write. All this work on DBI's part does come at a price - it adds a layer of interpretation and slows things down a bit. But you won't notice it witha flat-file until your user list approaches 1,000 or so rows.

2. Raw Perl. Here you go:

Suppose your user "database" is a flat-file that looks like this:

username|password|fname|lname|user_info
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Your html form in which you select the user to delete is going to have a drop down:

<form action="myscript.pl">
<select name="user">
<OPTION VALUE="uname_1">user name 1
<OPTION VALUE="uname_2">user name 2
</select>
</form>
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Then your Perl script is going to look like:

# use the CGI module, this automates processing of form input and take
+s care of a lot of security issues too
use CGI;
my $query = new CGI;
$user_to_delete = $query->param('user');

#load the user file into an array
open(FILE, '<user_data_file.txt');
@user_data = <FILE>;
close(FILE);

#to find the row with your user, cycle through the rows, parsing each 
+one and looking for the user name, and write all lines back to the us
+er database except the one for the user in question
open(FILE, '>user_data_file.txt');
for $i (0 .. $#user_data) {
     @this_user_info = split(/\|/, $user_data[$i]);
     if ($this_user_info[0] eq $user_to_delete) {
          #do nothing
     }
     else {
          print FILE "$user_data[$i]\n";
     }
}
close(FILE);
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This is very basic. You should add code for file locking, for example. And this could could be more elegant with the use of "unless" instead of the if-else.