#!/usr/bin/perl

use strict;
use Benchmark;

sub line_by_line {
        open (IN, "/usr/share/dict/words");
        while (<IN>){
                if ($_ =~ /hello world/){
                        # do what ever
                }
        }
        close IN;
}

sub block {
        open (IN, "/usr/share/dict/words");
        my @lines = <IN>;
        close (IN);
        foreach (@lines){
                if ($_ =~ /hello world/){
                        # do what ever
                }
        }
}

timethese(100,{ 'line' => \&line_by_line,
                'block' => \&block
              });

##</code><code>##

Benchmark: timing 100 iterations of block, line...
     block: 32 wallclock secs (32.52 usr +  0.36 sys = 32.88 CPU) @  3.04/s (n=100)
      line: 19 wallclock secs (17.92 usr +  0.16 sys = 18.08 CPU) @  5.53/s (n=100)