Of course, the spelled keywords 'and' and 'or' help de-parentheficate this sort of thing slightly:

if ( $a1 == $a2 and
     $a3 == $a4 and
     ($a5 == $a6 or $a7 == $a8) )
{
}
[download]

For readability, I also pull some tests into literate (readable, self-documenting) boolean names: my $nouns_match = ( $a3 == $a4 ). Then I can just refer to $nouns_match in the complicated conditional.