As an optimisation, closures only capture the variables they need. Perl has no idea what variable you will need for an eval EXPR (since EXPR can evaluate to anything), so those can't be taken into consideration.

If Perl made a mistake (as it does in the second function), it will let you know with the warning "Variable "$var" is not available".

The workaround is to make Perl notice you need the variable (like you did in the third function). Adding $var if 0; is a cheap and simple way of doing that without introducing warnings.