With such a description we can give only general advice (also smells a bit like homework), see perlrun, for, split and perlop:

/usr/bin/perl -i.bak

@pars= split /\n\n/, join '', <>;
for( @pars ){
  next if /pattern1/;
  print $newpara if /pattern2/;
  print;
  print "\n\n";
}
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Jeroen