Comment out the line with 'eval', and print it instead

#eval $code;
print $code;
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You might be surprised at the value of $code.

I'm sure you already know this, but I'll repeat it anyway - double quotes interpolate contained variables, single quotes do not. perl is trying to eval if ($port_a $cmp_op $port_b)... which is destined to fail - it won't even compile. In order to evaluate it properly, $code needs to to contain if ($port_a < $port_b)....

I'll leave it for you to construct the string properly ;) Once you do, comment the print and uncomment the eval.

BTW, what are you trying to accomplish? Could there be a better way?

Update: BTW, I didn't see your update until after posting this node. I assume most would see it easier if you posted a response instead the update.

Update #2 Update #1 is directed at the OP, not xenchu