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

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

use Tie::File;
tie my @lines, 'Tie::File', $filename
	or die "tie $filename: $!";
my $found;
for (my $i=0; defined($lines[$i]); $i++) {
	if ($lines[$i]=~/<!--\s*INSERT\s+HERE\s*-->/i) {
		$found=1;
		splice @lines, $i+1, 0, @to_insert;
		last;
	}
}
die "Marker not found" unless $found;
untie @lines;