my @lastmatches;
my $keep = 5;
while ( my $line = <FILEHANDLE> ) {
    next unless $line =~ /criteria/;
    push @lastmatches, $line;
    unshift @lastmatches if --$keep < 1;
}

##</code><code>##

my @lastfive = ( grep { /criteria/ } <FILEHANDLE> ) [ -5 .. -1 ];

##</code><code>##

use strict;
use warnings;

# Create the 5mb file.
my @alphabet = ( "A".."Z", "a".."z", " ", "\n");
open OUTFILE, ">file.txt" or die;
print OUTFILE $alphabet[ rand( @alphabet) ] for 1 .. (1024 * 1024 * 5);
close OUTFILE;

# Find the last five occurrences of 'abc'.
print "Testing grep method:\n";
open IN, "file.txt" or die;
my @lastfive = ( grep { /abc/ } <IN> ) [-5 .. -1];
close IN;
my $count = 5;
print $count--, ".: ", $_ foreach @lastfive;