use strict;
use warnings;

my $previous_line;
my $exclude = 0;

while (my $current_line = <DATA>)
{
    if (defined $previous_line)
    {
        if ($previous_line =~ /cat/ &&
            $current_line  =~ /dog/)
        {
            $exclude = 1;
        }
        else
        {
            emit_line($previous_line) unless $exclude;
            $exclude = 0;
        }
    }

    $previous_line = $current_line;
}

emit_line($previous_line) unless $exclude;

sub emit_line
{
    my ($line) = @_;

    print $line if defined $line;
}

__DATA__
dog
hippopotamus
cat 
dog
dog
cat
goat

##</code><code>##

17:11 >perl 1695_SoPW.pl
dog
hippopotamus
dog
cat
goat

17:11 >