First, you skip index 0. Second, if anyone modifies $[, your @a-1 won't be the last element. Anyhow, let's take a crack at it:

#!/usr/bin/perl -wl
use strict;

my @a = qw(1  2  1  1  2  2  1  2);
my @b = qw(aa aa ab ac ac ad ac ad);

print "first" if find_pair(1,"aa",\@a,\@b);
print "not printed" if find_pair(2,"ax",\@a,\@b);
print "not printed" if find_pair(4,"ax",\@a,\@b);
print "second" if find_pair(2,"ac",\@a,\@b);
print "third" if find_pair(2,"ad",\@a,\@b);

sub find_pair {
  my ($first,$second,$array1,$array2) = @_;
  $first eq $array1->[$_] && $second eq $array2->[$_] && return 1 for 
+$[..$#$array1
}

__END__
first
second
third
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