So the entire unless statement is not evaluated and the return is the last evaluation (your variable assignment).

This is not exactly true.

sub t1 {
    my $v = 'last expression';
    my $x = 'last assignment';
    unless ( $v ) {}
}
sub t2 {
    my $x = 'last assignment';
    unless ( 'last expression' ) {}
}
printf "t1 returns [%s]\n", t1();
printf "t2 returns [%s]\n", t2();
__END__
t1 returns [last expression]
t2 returns []
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Looking at the output from B::Deparse, It seems the first unless is left as-is, but the second one (where the condition is a literal) is reduced to !1 (which returns as the empty string).

If I change each unless to if, the second one reduces the same way, and the first one stays (just like the first unless stays originally).

So I think the rest of your email response is suspect also. In my testing, if seems to evaluate the same way as unless.