You're leaving out important steps. You need to 1) read in the file, 2) split it and put into an array ref, and 3) sort it.

for (sort third_field <INPUT>) {
  print "Record: $_";
}

sub third_field {
  my $x = (split ' ', $a)[2];
  my $y = (split ' ', $b)[2];
  $x <=> $y;
}
[download]

You might want to do a map-sort-map:

for (by_third_field <INPUT>) {
  print "Record: $_";
}

sub by_third_field {
  map $_->[0],
  sort { $a->[3] <=> $b->[3] }
  map { [ $_, split ] }
  @_;
}
[download]

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