The easy way is to look at B::Deparse and use the code that Perl writes for you in the background:

perl -MO=Deparse -lane "print $F[2]"
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BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = readline ARGV)) {
    chomp $_;
    our @F = split(' ', $_, 0);
    print $F[2];
}
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So, the line

    our @F = split(' ', $_, 0);
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basically is the -a command line switch.