I searched perldoc , deja, O'Reilly books, strict.pm but no luck.

You got your answer, but just a small comment - I don't think this information was so well hidden.

• From perldoc strict

  Because of their special use by sort(), the variables $a and $b are exempted from this check.

• The camel (3rd edition) has it in the index and this points to the section about the special variables. Furthermore you can find it in 'Pragmatic Modules'->use strict 'vars'

-- Hofmator