It won't work on general data. It will find the first subsequence of @a that is in @b and be content with it. But it was an interesting attempt.

Update: For @a = qw(a b c); @b = qw(a b c) it prints b. For two element lists it does not work at all.

Update: Changed the code to:

my $str = join(" ", @a) . "&" . join(" ", @b);

if ($str =~ /(?:\b\w+\b\s*)*?
             ((?:\b\w+\b\s*)+)
             (?:\s*\b\w+\b\s*)*?
             &
             (?:\b\w+\b\s*)*?
             \1
             (?:\s*\b\w+\b)*?/x) {
    my $result = $1;
    $result =~ s/\s*$//;
    print "found '$result'\n";
}
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It does not have the problem with cutting the first and last elements of the list, but still for @a = qw(a b c); @b = qw(a x b c) it prints a.