open (FILEHANDLE2, "<&=FILEHANDLE");

    

open FILEHANDLE, "<file.txt" or die "Can not open file";

open (FILEHANDLE2, "<&=FILEHANDLE");

while (<FILEHANDLE2>) {
    print;
}

1.	To open a file for appending:
2.	To open a file for reading and writing:
3.	To open a file for reading:
4.	To open a file for writing
5.	open (FILEHANDLE, "<&FILEHANDLE2");
6.	open(MYFILE, ">>c:\\outfile.dat")
7.	Open FILEHANDLE
8.	Open Operators
9.	Open a file
10.	Open a file from another file handle
11.	Open a file to read
12.	Open a pipe to the who command
13.	Open command expressions
14.	Open file for update
15.	Open file for writing
16.	Open file reading
17.	Open for Appending
18.	Open for Reading
19.	Open for Reading and Writing
20.	Open for Writing
21.	Open for writing first, then reading
22.	Opening an input filter on a Win32 platform
23.	Opening files and using file handles: open(filehandle, name);
24.	Opens two files and copies one into another.
25.	Checks whether an unopened file actually exists.
26.	Input Filter: open(FILEHANDLE, COMMAND\|);
27.	Sending the Output of a Filter to a File: open(STDOUT, ">/dev/tty");
28.	Uses die when testing for a successful file open operation.
29.	Using open to connect different processes

open (FILEHANDLE2, "<&=FILEHANDLE"); : Open file « File « Perl