Hang on, not so fast. ;-)

Your regular expression will only find matching occurances of XXX ... YYY, *NOT* to validate all occurances.

To validate a sequence, you will have to make sure there is no mismatches of XXX, and hence require a bit more work.

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
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