#!/usr/bin/perl 

die "Usage: $0 <N>, where N is the number of lines to pick\n" 
    if @ARGV < 1; 
 $N = shift @ARGV; 

@pick = (); 
 while (<>) { 
    if (@pick < $N) { 
      push @pick, $_; 
      ($r1, $r2) = (rand(@pick), rand(@pick)); 
      ($pick[$r1], $pick[$r2]) = ($pick[$r2], $pick[$r1]); 
    } else { 
      rand($.) <= $N and $pick[rand(@pick)] = $_; 
    } 
 } 

print @pick;