Why do you expect zero specifically?

The false value typically returned by Perl is a special value that's "" when treated as a string and 0 when treated as a number.

$ perl -wle'my $x = !1; print "".$x; print 0+$x;'

0

$ perl -wle'my $x = ""; print "".$x; print 0+$x;'

Argument "" isn't numeric in addition (+) at -e line 1.
0

$ perl -wle'my $x = 0; print "".$x; print 0+$x;'
0
0
[download]