Answer

In Perl, filehandles are special objects (global or lexical variables) that are associated with a physical or virtual file. It is very important to properly open, check, and close files to prevent resource leaks and security violations.

Example of proper handling:

open my $fh, '<', 'myfile.txt' or die "Can't open file: $!";
while (my $line = <$fh>) {
    print $line;
}
close $fh or warn "Couldn't close file: $!";

Details:

• Files should be opened in modern Perl5 mode: use lexical filehandles (open my $fh, ...). Global filehandles (OPEN FILE, ...) are considered deprecated.
• Checking the success of opening a file is MANDATORY. Always use or die/or warn.
• After working with a file, it should be closed using close and handle possible errors.
• For binary files, use layers: binmode $fh.
• To avoid vulnerabilities, always escape variables in paths and check access rights.

Trick Question

What is the difference between the open FH, ... construct and open my $fh, ..., and why is the second option recommended?

Answer: The open my $fh, ... construct creates a lexically scoped filehandle, which means it only works within the current block/subroutine and is automatically destroyed outside of its scope. This prevents accidental overriding of filehandles, protects against global filehandle name conflicts, and improves reliability.

open FH, '<', 'file.txt';    # Global filehandle! Recommended to avoid.
open my $fh, '<', 'file.txt'; # Lexical, safe.

Examples of real errors due to lack of knowledge about the subtleties of the topic

---

Story

In one large project, the use of identical global filehandles (OPEN LOG, ...) was permitted. As a result, parallel requests accidentally wrote to the wrong files, leading to loss or mixing of logs.

---

Story

A developer forgot to check for the successful opening of a file; as a result, if the file did not exist, an error was logged elsewhere (for example, logs were written incorrectly, other people's files were deleted, etc.).

---

Story

The absence of a close call led to exhaustion of filehandles on the server during mass file processing — the server began to "hang" and crash with huge amounts of data.