• Type: Variables are either scalars prefixed by $, arrays of scalars prefixed by @, or hashes of scalar pairs prefixed by %.

• Operators: same as Java except eq, gt, lt, … for string comparisons.

• Control Structures (selection and iteration): same as Java plus foreach for hashes.

• Cool stuff: interpolation, here-docs, regex, and web goodies such as param and %ENV.

Default file handlers:

• STDIN: Standard input from console
• STDOUT: Standard output to console
• STDERR: Standard error output to console

Opening a file

open(FILEHANDLE, filename)

Where FILEHANDLE specifes the name for the file handler in which you will refer to your file and filename specifies the location of your file.

In the file name one can specify the various read/write modes.

File locking for I/O is provided using the flock() function. It is suggested to do the lock on a seperate file to prevent race conditions.

open(LOCK, ">write.lock")
flock(LOCK, LOCK_EX);
open(DATA, myFile);
#...do work with the DATA file...
close LOCK;
close DATA;

Closing the LOCK handler automatically unlocks the semaphore. One can also use flock(LOCK, LOCK_UN) to do the unlocking specifically. The flock function takes the following operation parameters:

So by now we know what an array looks like and maybe we've learned a few ways to manipulate it. Well, here's a trick to remove duplicate items in an array while preserving the item's order by using a hash.

Here's a couple of things you ought to know to fully understand the code:

• A hash is like an arrays except that they link a key to a value and the key is unique.
• $_ is a special perl variable known as the default or implicit variable. In our example below it'd be the implicit value of the current @array1 item.

@array1 = qw(one two one three four two);
 
foreach (@array1) {
    push(@array2, $_) unless ($seen{$_}++);
}
 
print "@array2";

output: one two three four

The reason that we're using a hash here is to use its limitation (each key must be unique) to our advantage. In this example we use a hash called “seen” with keys set to each item in our array. For each string we test to see if that key already refers to a non-false value. The post-increment operator (++) adds one to that key's value, but only after the condition is checked. The result is that the first time a key string is encountered the condition is false (since the value is undefined) and the key is therefore pushed onto our second array. The key is then incremented and any further test of the key's value will evaluate “true” (i.e. non-zero, and non-empty).

Reference: http://www.perlcircus.org/arrays.shtml