use strict;

my $data;
{
    local $/;
    $data = <DATA>;
}

# number of times didn't match XXX ... YYY
# is total number of XXX minus the XXX ... YYY
# pattern appears.

my $non_match =
    $#{[$data =~ /(XXX)/gs]} -
    $#{[$data =~ /(XXX\s*\n+YYY)/gs]};

print !$non_match ?
      "All sequences of XXX - YYY are ok\n" :
      "$non_match bad sequences found\n";

__DATA__
XXX
YYY
XXX
ZZZ
XXXYYY