eval 'print OUT "$unsafe_input" or die';

I do not think this is going to do what you said it does. Variables are not interpolated inside 'single quotes', so the eval only interpolates the variable one time. So, even if $unsafe_input='$DATABASE_PASSWORD', the password would not be printed.

On the other hand, it would print the password if the code was like this: eval "print OUT \"$unsafe_input\" or die";