结论

• $var//=2：等价于 defined($var)||2，即 未定义 时才赋值为 2 ，否则不变（ 即使是 或 空字符串 ）
• $var||=2 ：除非定义且为 true 才不会赋值，否则赋值（比如 或 空字符串 时）为2。

//=

Step-1 空串

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

# perl atest4.pl 
''

Step-2 0

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

# perl atest4.pl 
'0'

Step-3 1

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

# perl atest4.pl 
'1'

Step-4 undef

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

# perl atest4.pl 
'2'

||=

Step-1 空串

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

# perl atest4.pl 
2

Step-2 0

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

# perl atest4.pl 
2

Step-3 1

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

# perl atest4.pl 
1

Step-4 undef

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

# perl atest4.pl 
2

-r: File is readable by effective uid/gid.
-w: File is writable by effective uid/gid.
-x: File is executable by effective uid/gid.
-o: File is owned by effective uid.

-R: File is readable by real uid/gid.
-W: File is writable by real uid/gid.
-X: File is executable by real uid/gid.
-O: File is owned by real uid.

-e: File exists.
-z: File has zero size (is empty).
-s: File has nonzero size (returns size in bytes).

-f: File is a plain file.
-d: File is a directory.
-l: File is a symbolic link.
-p: File is a named pipe (FIFO), or Filehandle is a pipe.
-S: File is a socket.
-b: File is a block special file.
-c: File is a character special file.
-t: Filehandle is opened to a tty.

-u: File has setuid bit set.
-g: File has setgid bit set.
-k: File has sticky bit set.

-T: File is an ASCII text file (heuristic guess).
-B: File is a "binary" file (opposite of -T).

-M: Script start time minus file modification time, in days.
-A: Same for access time.
-C: Same for inode change time (Unix, may differ for other platforms)

参考文献

• Alphabetical Listing of Perl Functions
• [What does if( -f ) in Perl do?](https://stackoverflow.com/questions/6172957/what-does-if-f-filename-in-perl-do)
• perl中 -e, -z, -s, -M, -A, -C, -r, -w, -x, -o, -f, -d , -l 含义