#!/usr/bin/perl

use strict;
use warnings;

use CGI ();
my $cgi = CGI->new();

my $infile = '/path/to/file.html';

open my ($fh), '<', $infile or die "Couldn't open $infile! $!";
read $fh, my ($html), -s $infile or die "Couldn't read $infile! $!";
close $fh or die "Couldn't close $infile! $!";

my $generated_password = function_to_generate_password();

$html =~ s/some_special_target_in_your_html_file/$generated_password/sgi;

print $cgi->header();
print $html;