Check out the Benchmark module (perldoc Benchmark). Here's what it says:

#!/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
              });
[download]

Which Yields

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)
[download]

/\/\averick
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"

Updated:Fixed regexp to be the same in both subs. Used question's code verbatim. Thanks lemming.