Understand the concept of file handles

Perl
File input/output
here

The concept of file handles in Perl is confusing, so I'll explain it here.

File handles are conceptual rather than real

When we talk about file handles in Perl, we mean multiple entities that Perl recognizes as file handles. There is no such thing as a file handle.

What Perl recognizes as a filehandle

Symbol (FH)
Type glob(* FH)
Reference to typeglobs (\ * FH)
IO::Handle class object
An object of a class that inherits the IO::Handle object (such as IO::File)

Perl recognizes the above five as file handles. following statement are all valid.

# 1. Symbol FH
open(FH, "<", $file) or die "$!";

# 2. Type glob * FH
open(* FH, "<", $file) or die "$!";

# 3. Reference to typeglobs\* FH
my $fh =\* FH;
open($fh, "<", $file) or die "$!";

# 4. IO::Handle object
use IO::Handle;
my $fh = IO::Handle->new;
open($fh, "<", $file) or die "$!";

# 5. IO::File object (inheriting IO::Handle)
use IO::File;
my $fh = IO::File->new;
open($fh, "<", $file) or die "$!";

How to write a modern file open

open(my $fh, "<", $file)
  or die "Can't open file $file:$!";

is.

When an undefined a scalar variable is passed as the first argument to the open function, a reference to the anonymous glob is automatically generated. With the same description as "3. Reference to typeglob(\ * FH)", only the part of my $fh =\* FH; is automatically performed. (This is a modern implementation of Perl, which may in the future automatically generate objects to IO::Handle.)

Perl ABC