gistfile1.pl

use File::Copy;

copy("file.html","file.html.orig");

open(my $out, ">",  "file.html.tmp");
open(my $in,  "<",  "file.html");

for ( @lines ) {
    s/$pattern/${replacement}\n$pattern/g;
    say $out $_; #########################################################
	# you have to print to the file
}

# then copy the output to the proper file name

copy("file.html.tmp", "file.html");