if @array =~ $elem {...}        # true if @array contains $elem

    if $key =~ %hash {...}          # true if %hash{$key}

    if $value =~ (1..10) {...}      # true if $value is in the list

    if $value =~ ('a',/\s/,7) {...} # true if $value is eq to 'a'
                                    #   or if $value contains whitespace
                                    #   or if $value is == to 7