File Handling

Introduction

Perl was built for chewing through text files, so its file-handling tools are simple, direct, and battle-tested. This lesson covers the modern, safe way to open files, read them (both line by line and all at once), write new content, and append to existing files.

The Three-Argument open

open takes a filehandle (stored in a lexical scalar variable), a mode, and a filename, as three separate arguments. This modern "three-argument open" form is safer than the older two-argument form because it treats the filename as pure data, avoiding surprises if a filename happens to contain special characters.

use strict;
use warnings;

my $filename = "notes.txt";

open(my $fh, '>', $filename) or die "Could not open '$filename': $!";
print $fh "First line of notes.\n";
close($fh);

print "File written successfully.\n";

Reading a File Line by Line

The most idiomatic way to process a text file in Perl is to open it for reading and loop over the filehandle with while, which reads one line at a time. This keeps memory usage low even for very large files, since only one line is held in memory at once.

use strict;
use warnings;

my $filename = "notes.txt";

open(my $fh, '<', $filename) or die "Could not open '$filename': $!";

my $line_number = 0;
while (my $line = <$fh>) {
    $line_number++;
    chomp $line;
    print "Line $line_number: $line\n";
}

close($fh);

chomp removes the trailing newline character from $line, if there is one. Without it, every printed line would carry an extra blank line from the original file's line ending.

Reading an Entire File at Once

Sometimes it is more convenient to slurp the whole file into a single array (one element per line) or a single scalar (the whole content as one string), especially for smaller files.

use strict;
use warnings;

my $filename = "notes.txt";

open(my $fh, '<', $filename) or die "Could not open '$filename': $!";
my @all_lines = <$fh>;
close($fh);

print "The file has ", scalar(@all_lines), " line(s).\n";
print "First line: $all_lines[0]";

Writing to a File

Opening with mode > and then using print (or printf) with the filehandle sends output to the file instead of the screen. Be aware that > always overwrites the file completely if it already exists.

use strict;
use warnings;

my @lines_to_write = ("Alice,29", "Bob,34", "Carol,41");

open(my $fh, '>', 'people.csv') or die "Could not open file: $!";
foreach my $line (@lines_to_write) {
    print $fh "$line\n";
}
close($fh);

print "Wrote ", scalar(@lines_to_write), " lines to people.csv\n";

Appending to a File

Mode >> opens the file for writing without erasing existing content — new output is added to the end. This is the mode you want for something like a running log file.

use strict;
use warnings;

open(my $fh, '>>', 'people.csv') or die "Could not open file: $!";
print $fh "Dave,25\n";
close($fh);

open(my $read_fh, '<', 'people.csv') or die "Could not open file: $!";
my @lines = <$read_fh>;
close($read_fh);

print "people.csv now has ", scalar(@lines), " line(s).\n";

Closing Filehandles

Calling close explicitly when you are done with a filehandle flushes any buffered output to disk and frees the underlying file descriptor. In modern Perl, a lexical filehandle (my $fh) is also closed automatically once it goes out of scope, but explicit close calls make your intent clear and let you check for a rare close-time error.

use strict;
use warnings;

open(my $fh, '>', 'important.log') or die "Could not open file: $!";
print $fh "Critical data\n";
close($fh) or warn "Could not close file properly: $!";

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

File handling lets Perl scripts read configuration, process logs, and produce reports — exactly the kind of automation Perl was originally built for. Next, you will learn how scripts can receive input directly from the command line itself, via @ARGV.