$ perl -Mstrict -wE ' my $x; say "\$x not defined: ", (! defined $x) ? "Y" : "N"; say "\$x not TRUE: ", (! $x) ? "Y" : "N"; $x = 0; say "\$x not defined: ", (! defined $x) ? "Y" : "N"; say "\$x not TRUE: ", (! $x) ? "Y" : "N"; $x = 1; say "\$x not defined: ", (! defined $x) ? "Y" : "N"; say "\$x not TRUE: ", (! $x) ? "Y" : "N"; ' $x not defined: Y $x not TRUE: Y $x not defined: N $x not TRUE: Y $x not defined: N $x not TRUE: N