can anyone tell me what this code is trying to-do

It's trying to confuse you. Looks like it succeeded.

Running it though B::Deparse gives the following, which might be a bit clearer.

BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    {
        ++$_;
        --$_;
        $_ ||= 4;
        if ($_ >> 2 or $_ << 2 > 12) {
            $_ = 'Vainyvq ragel';
            &{$F[0];};
            last;
        }
        &t;
        $0 -= $_;
        $_ = 'Lbh jva';
        die &{$F[0];} if not $0 - 1;
        $0 -= $0 % 2 ? ($0 > 2 ? 2 : 1) : ($0 <= 5 ? ($0 > 2 ? 3 : 1) 
+: (rand > 0.5 ? 1 : 3));
        $_ = 'V jva';
        die &{$F[0];} if not $0 - 1 > 1;
    }
    &v;
}
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