Reading the file into an array might be a big mistake, instead, use the in-place editing:

local ($^I,@ARGV)=('.bak','file.html');
my $p=1;
while(<>){
$p-- if /<\/tr>/;
print "Add this line\n" if $p==-4
print if $p;
$p-- if /<tr>/;
}
[download]

You can read more about in-place editing in perlvar($^I) and perlrun(-i).