I would also mention just a simple ternary operator as an alternative: 
return grep($_ eq $_[0], @_[ 1 .. $#_ ]) ? 1 : '';

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or, more generally the equivalent to !!$expr is (some of the parens are optional depending on context): 
(($expr)?1:'')

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In reply to Re^2: Perl Idioms Explained - !!expr by davidrw
in thread Perl Idioms Explained - !!expr by broquaint

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