Well in a lot of cases //x isn't useful... *but*...

$s =~ /(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA|
        BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB|
        CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC)/x;
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I've personally had horrendously huge regex's that either were going to be about 3,000 characters on one line or I had to break them up to multiple lines. /x is the only way you can do this without picking up spurious white space and carriage returns and still make things look nice. yes you have to escape any spaces you have, but that's the price you have to pay for readability and maintainability sometimes!