$/ = undef;
$filename = "$root/content/topics/$get{'topic'}";

open( TOPIC, "+<", $filename ) or die "Unable to read/write $filename\n";
$_ = <TOPIC>;

if( /^\Q$get{'post'}\E/ ) {
   s/.*\n$/$post/m;
}

seek( TOPIC, 0, 0 );   # rewind to start of file
print TOPIC;
close TOPIC;