If you really want efficiency, have multile loops rather than a single loop with an if test. Like this: 
use strict;
my $size = shift @ARGV || 3; # sample size
my @sample;
# read n lines
for (1..$size) {
  push @sample, scalar <>;
}
# shuffle - I'll do it in pure perl.
my $i = $size;
while ($i--) {
  my $j = int rand($i + 1);
  @sample[$i, $j] = @sample[$j, $i];
}
# now read the rest of the lines in.
while (<>) {
  my $choice = int rand($.);
  if ($choice < $size) {
    $sample[$choice] = $_;
  }
}
# and minimize work by chomping as few lines as possible.
chomp(@sample);

[download]
This can be beaten. But not by much...

Update: Per the note by AM below, fix off-by-one error.

In reply to Re: selecting N random lines from a file in one pass by tilly
in thread selecting N random lines from a file in one pass by seuratt

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