Re^2: Perl Idioms Explained - !!expr

[broquaint]

  return grep { $_ eq $_[0] } @_[ 1 .. $#_ ];

Depending on context this will return different things, whereas !!expr will always return a boolean value.

  return ~~grep { $_ eq $_[0] } @_[ 1 .. $#_ ];

This will return the number of elements returned by grep as opposed to !!expr which, again, will return a boolean value.

Of course both of the above are valid in appropriate situations, but if you just want a boolean value then !!expr is an appropriate idiomatic solution.