How to open a file for appending in Perl?

How to Open a File for Appending in Perl

In Perl, opening a file for appending means you want to add new data to the end of an existing file (or create the file if it doesn't exist) without overwriting its current contents. This is a common task for logging, data collection, or incremental updates.

Perl’s open built-in function is used to open files with various modes. To open a file for appending, you use the append mode by specifying the mode string '>>' when opening the file handle. The double angle brackets >> instruct Perl to open the file for appending.

Basic Syntax for Opening a File in Append Mode

open(my $fh, '>>', 'filename.txt') or die "Cannot open filename.txt: $!";

• $fh is a lexical filehandle (recommended over traditional bareword handles).
• '>>' is the mode that means append.
• 'filename.txt' is the path to your file (can be relative or absolute).
• die prints an error message if the file cannot be opened and terminates the program.

Complete Runnable Example

#!/usr/bin/perl
use strict;
use warnings;

# Open the file 'log.txt' for appending
open(my $log_fh, '>>', 'log.txt') or die "Cannot open log.txt for appending: $!";

# Print a timestamped log entry to the file
my $timestamp = localtime();
print $log_fh "Log entry at $timestamp\n";

# Close the filehandle explicitly (good practice)
close($log_fh) or warn "Could not close log.txt: $!";

# Print confirmation to STDOUT
print "Appended a log entry to log.txt\n";

This code will either create log.txt if it doesn’t exist or append the timestamped message at the end of this file. You’ll see a confirmation on standard output.

Some Useful Notes and Gotchas

• File modes: The modes for open are:
  • < for reading
  • > for writing (truncates file)
  • >> for appending
  • Optional : can be used to specify encoding layers (e.g., ':encoding(UTF-8)') from Perl 5.8 onwards.
• Use lexical filehandles: Using open(my $fh, ...) is safer and cleaner than older bareword handles like open(FILE, ...).
• Autoflush: If you’re appending logs and want immediate writes, consider setting autoflush with $fh->autoflush(1); (requires use IO::Handle;), but not necessary in simple scripts.
• File permissions: If the file doesn't exist, it will be created with your system's default permissions (typically 0666 modified by umask). To specify permissions on creation, use the sysopen function.
• Always check open success: Using or die (or or warn) ensures your script doesn’t silently fail if the file cannot be opened (e.g., due to permissions or disk errors).

Summary

To append to a file in Perl, just use:

open(my $fh, '>>', 'file.txt') or die $!;
print $fh "Appended text\n";
close $fh;

It’s a simple yet robust pattern for file output when you want to preserve existing content and add more data safely.