#!/usr/bin/perl -wT
use strict;

# position to sort by
my $sortbyfield = 3;
my @input = (
        '1|2|1|4|5|6|7|8|9',
        '1|2|2|5|5|6|7|8|9',
        '1|2|0|1|5|6|7|8|9',
        '1|2|9|2|5|6|7|8|9',
        '1|2|8|3|5|6|7|8|9',
        '1|2|7|4|5|6|7|8|9',
        '1|2|6|7|5|6|7|8|9',
);

# Schwartzian Transform
my @output =
  map { $_->[0] }
  sort { $a->[1] <=> $b->[1] }
  map { [$_, getfieldvalue($_, $sortbyfield)] }
  @input;

print join( "\n", @output), "\n";

sub getfieldvalue {
        my ($line, $fieldno) = @_;
        my @items = split /\|/, $line;
        return $items[$fieldno];
}
__END__
returns (sorted by 3rd _position, arrays start at 0)

1|2|0|1|5|6|7|8|9
1|2|9|2|5|6|7|8|9
1|2|8|3|5|6|7|8|9
1|2|1|4|5|6|7|8|9
1|2|7|4|5|6|7|8|9
1|2|2|5|5|6|7|8|9
1|2|6|7|5|6|7|8|9