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  What is "/o" really for?

...
        while (<>) {
            print if /$pat/o;
        }

#!/usr/bin/perl -w

...
            noPrecomp => \&noPrecomp
         }
     );

Working from pattern 'a|ab|abc|abcde|ef|gh|ghij|qrst|nqmz|stuv', qr='(
+?-xism:a|ab|abc|abcde|ef|gh|ghij|qrst|nqmz|stuv)'
preComp finds 6
...
noPrecomp 33414/s        --       -5%       -6%
preCompQR 35094/s        5%        --       -1%
preComp   35360/s        6%        1%        --