use Tie::File;
use Fcntl 'O_RDONLY';
my @array;
tie @array, 'Tie::File', $ARGV[0], mode => O_RDONLY or die;
foreach my $i ( 0 .. $#array ){
  # no, doesn't handle index edge case of $i < 4
  print join "", @array[$i-4,$i-1] if $array[$i] =~ /foo/;
}