Here's how to do it without any eval at all. As a plus, you get compile-time syntax checking:

sub Instance_Var ($) {
    my $package=__PACKAGE__;
    my ($var)=@_;
    my $accessor_name = "$package\::$var";
    my $acc = sub {
        my $t = shift;
        $t->{$var}=$_[0] if @_;
        $t->{$var};
    };

    # Now install our anonymous subroutine as $package::$var
    no strict 'refs';
    *{ $accessor_name } = $acc;
}
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You might also want to see Class::Accessor and any of the other Accessor modules. Maybe also Class::Data::Inheritable.