use File::Replace 'replace2';
 
my ($infh,$outfh) = replace2($filename);
while (<$infh>) {
    # write whatever you like to $outfh here
    print $outfh "X: $_";
}
close $infh;   # closing both handles will
close $outfh;  # trigger the replace

##</code><code>##

#!/usr/bin/env perl
use warnings;
use strict;
use File::Replace 'replace2';

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

my ($ifh,$tfh) = replace2($filename);
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;
die "Marker not found" unless $found;