That doesn't make immediate sense to me. The only thing that comes to mind is the possibility that you're looping thousands and thousands of times, and there's nothing in $rtxt for most of those times. Hash autovivification could be creating new memory structures for you. Something like this could help:

if (exists $rDTX->{$key}{$txqu}{$txit}{ResponseText}) {
    print $rDTX->{$key}{$txqu}{$txit}{ResponseText}, "\n";
}
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It's a little longer to type, but exists skips the autovivification. Depending on what's defined in $rDTX and where, you may have to see if more keys exist (like $txit, $txqu, and $key). It's hard to tell without seeing example data, though.