$ perl -wMstrict -le 'my $x=""; print 3+$x;'
Argument "" isn't numeric in addition (+) at -e line 1.
3
$ perl -wMstrict -le 'my $x=!!""; print 3+$x;'
3
$ perl -wMstrict -le 'my $x=defined(); print 3+$x;'
3