rpick.pl

#! perl

use strict;
use warnings;

my $lines_to_pick = 5; # 何行選ぶか
my $lines_total = 0;

my @picked;

while (my $line = <STDIN>) {
    ++$lines_total;
    if (@picked < $lines_to_pick) {
        push @picked, $line;
    } elsif (rand() < $lines_to_pick / $lines_total) {
        $picked[int(rand() * $lines_to_pick)] = $line;
    }
}

print $_ for @picked;