#/usr/bin/perl
use strict;
use warnings;

my @lines;

while (my $cur_line = <DATA>)
{
   push @lines, $cur_line;
   next if @lines <2;
   
   print @lines unless ($lines[0] =~ /dog/ and $lines[1] =~ /cat/);
   @lines = ();
}
=prints
dog
dog
cat
goat
=cut

__DATA__
dog
cat
dog
dog
cat
goat