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use strict;
use warnings;
...
if (/((A(?:B(?:C(?:\s+(?:X(?:Y(?:Z)?)?)?)?)?)?))/) {
    print "Searched for 'ABC\\s+XYZ' but only found $1";
}

use strict;
use warnings;
...
print "Searched for 'ABC\\s+XYZ' but only found '$longest'\n";
__END__
Searched for 'ABC\s+XYZ' but only found 'ABC X'