It's a bug in Rakudo that it doesn't reject the program at compile time, because $var isn't declared (yet). It's one of our oldest open bugs.

Update: It should be noted though that lexical variables are always scoped to a block in Perl 6, and that the detection of of undeclared variables happens only at compile time. Thus this piece of code works differently in Perl 5 and Perl 6 (and this time Rakudo is correct here):

$ perl -Mstrict -wE 'sub f { say eval q[$x] }; my $x = 5; f'
Use of uninitialized value in say at -e line 1.

$ perl6 -e 'sub f { say eval q[$x] }; my $x = 5; f'
5
$
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Here the $x is scoped to the whole mainline (which is an implicit block), so an eval sees it at runtime even though the eval comes textually before the declaration of $x.

Second update: I've fixed the bug, so updating to the latest development version should give the desired behavior.

The fix is a bit incomplete, but you're now much more unlikely to come across this problem.