Don’t worry if you don’t know what some of the other file tests mean—if you’ve never heard of them, you won’t be needing them. But if you’re curious, get a good book about programming for Unix. (On non-Unix systems, these tests all try to give results analogous to what they do on Unix, or give undef for an unavailable feature. Usually, you’ll be able to guess what they’ll do.)
If you omit the filename or filehandle parameter to a file test (that is, if you have-ror just-s), the default operand is the file named in$_.* So, to test a list of filenames to see which ones are readable, you type the following:
foreach (@lots_of_filenames) { print "$_ is readable\n" if -r; # same as -r $_ }
But if you omit the parameter, be careful that whatever follows the file test doesn’t look like it could be a parameter. For example, if you wanted to find out the size of a file in KB rather than in bytes, you might be tempted to divide the result of-sby1000(or1024), like this:
# The filename is in $_ my $size_in_K = -s / 1000; # Oops!
When the Perl parser sees the slash, it doesn’t think about division. Since it’s looking for the optional operand for-s, it sees what looks like the start of a regular expression in forward slashes. To prevent this confusion, put parentheses around the file test:
my $size_in_k = (-s) / 1024; # Uses $_ by default
Explicitly giving a file test a parameter is safer.
Perl Programming 
