#!/usr/bin/env perl
use warnings;
use strict;

use File::Temp qw/tempfile/;
use File::Basename qw/fileparse/;

my $filename = "/tmp/test.html";
my @to_insert = ( '<p>Hello,',
	'World! It is '.gmtime(time).' UTC</p>' );

open my $ifh, '<', $filename or die "open $filename: $!";

my ($basename,$path) = fileparse($filename);
my ($tfh,$tfn) = tempfile( $basename.'_XXXXXXXXXX',
	SUFFIX=>'.tmp', DIR=>$path );
#warn "Debug: writing to $tfn\n";

# change the lines of the input file however you wish here
my $found;
while (<$ifh>) {
	print $tfh $_;
	if (/<!--\s*INSERT\s+HERE\s*-->/i) {
		$found=1;
		print $tfh "$_\n" for @to_insert;
	}
}

close $ifh;
close $tfh;
chmod(0666&~umask,$tfn) or warn "Couldn't chmod $tfn: $!";
if (!rename($tfn, $filename)) {
	my $e=$!;  unlink($tfn);
	die "Renaming $tfn to $filename failed: $e";
}
#warn "Debug: Renamed $tfn to $filename\n";
die "Marker not found" unless $found;