my $file="test.dat";
my $insert="foo";

{
 local $/ = undef;
 open(my $fh, '<', $file) or die "$!";
 my $data = <$fh>;
 
 $data=~s/<\/body>/$insert\n<\/body>/;

 open(my $fh,'>',$file) or die "$!";
 print $fh $data;
 close($fh);
}