First, make sure you do

use strict;
use warnings;
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in your code if you aren't already.

To be honest, I have not looked into this in detail, but have a look at the $code =<<"END" part. The double quotes will make your string be interpolated, which mean while assigning to $code your variables $dfv etc will be expanded, and may well be undefined. Thus eval might see only my = shift;, which doesn't compile. eval fails and sets $@ aka $EVAL_ERROR, which you should check for.

Use single quotes around END, or escape everything that might be interpolated.

Also, you might consider moving the semicolon after the END up, that is:

$code = <<'END';
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I hope I got it somewhat right, and it helps a bit.