if (defined(my $foo = exists $lookup{$bar})) {
    ....
}

##</code><code>##

if (exists $lookup{$bar}) {
    my $foo = $lookup{$bar};
    ...
}

##</code><code>##

for my $foo (sort keys %some_hash) {
    my $by_foo = $some_hash{$foo};
    for my $bar (sort keys %$by_foo) {
        my $by_bar = $by_foo->{$bar};
        for my $baz (sort keys %$by_bar) {
            ...
        }
    }
}