#!/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
              });