$ perl -lwe 'for (qw/undef 0 "" !1 1==2/) { print "\n\$a = eval $_;"; $a = eval $_; print "$_:\t|$a|", 1+$a; }'

$a = eval undef;
Use of uninitialized value in concatenation (.) or string at -e line 1.
Use of uninitialized value in addition (+) at -e line 1.
undef:  ||1

$a = eval 0;
0:      |0|1

$a = eval "";
Argument "" isn't numeric in addition (+) at -e line 1.
"":     ||1

$a = eval !1;
!1:     ||1

$a = eval 1==2;
1==2:   ||1