$ perl -Mstrict -wE '
> sub grok
> {
>     my $str = shift;
>     say
>        defined $str
>        ? $str
>          ? qq{>$str<: defined and true}
>          : qq{>$str<: defined but not true}
>        : qq{>$str<: undefined};
> }
> 
> my $str;
> grok( $str );
> $str = q{};
> grok( $str );
> $str = q{   };
> grok( $str );
> $str .= q{ABC};
> grok( $str );'
Use of uninitialized value $str in concatenation (.) or string at -e line 5.
><: undefined
><: defined but not true
>   <: defined and true
>   ABC<: defined and true
$