$var='';
$var//=2;
print "'$var'\n";

# perl atest4.pl 
''

$var=0;
$var//=2;
print "'$var'\n";

# perl atest4.pl 
'0'

$var=1;
$var//=2;
print "'$var'\n";

# perl atest4.pl 
'1'

$var=undef;
$var//=2;
print "'$var'\n";

# perl atest4.pl 
'2'

$var='';
$var||=2;
print $var;

# perl atest4.pl 
2

$var=0;
$var||=2;
print $var;

# perl atest4.pl 
2

$var=1;
$var||=2;
print $var;

# perl atest4.pl 
1

$var=undef;
$var||=2;
print $var;

# perl atest4.pl 
2