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@data = (
  'buy 1/23/01 100 12.625 50 25.25',
...
  'buy 10/23/01 100 12.625 50 25.25',
  'buy 10/25/01 100 12.625 50 25.25',
);

sub get_date {
  my $string = shift;
  my $date = (split ' ', $string)[1];  # second field
  return $date;

for $line (@data) {
  push @dates, get_date($line);
}

for $line (@data) {
  push @dates, [ get_date($line), $line ];
}

sub get_date {
  my $string = shift;
...
  my ($d, $m, $y) = split '/', $date;
  return sprintf "%02d%02d%02d", $y, $m, $d;
}

@dates = sort { $a->[0] <=> $b->[0] } @dates;

@data = map $_->[1], @dates;

@data =
  restore(
    sort { $a->[0] <=> $b->[0] }
      extract(@data)
  );

# extract(@data)
# becomes
map [ get_date($_), $_ ], @data

# restore(...)
# becomes
map $_->[1], ...

@data =
  map $_->[1],
  sort { $a->[0] <=> $b->[0] }
  map [ get_date($_), $_ ],
  @data;

@data =
  map  { restore($_) }
  sort { ... }
  map  { extract($_) }
  @data;