The most common use of the chomp function is to remove trailing newlines from strings. Either when reading from the Standard Input (STDIN), or when reading from a file like this:

my $input = <STDIN>;
chomp $input;

while (my $row = <$fh>) {
    chomp $row;
    ...
}

This is not the whole storry though.

When dealing with a file, it can be either binary or text file. We usually consider files that have "lines" to be text files. When we read such file we usually do it line-by-line. We know that different operating systems have different meaning of what a new-line is, the most common ones use either a single LF - line feed (hexa 0x0A or decimal 10) character (OSX/Linux/Unix), or a CR - carriage return (hexa 0x0D or decimal 13) followed by a LF - line feed (MS Windows).

So when we open a text-file for reading and we call the read-line operator in scalar context: $line = <$fh> Perl will know what to do. Perl will adapt itself to the environment and will know what is the new-line symbol in the current operating system.

In order to do this, Perl maintains a variable called the Input Record Separator. In native Perl this is the $/ variable, but if you use the >English module you can use the name $INPUT_RECORD_SEPARATOR or $RS as well. This variable contains the characters that mean "new-line" in the current operating system.

That is, it will hold a LF character when running on OSX/Unix/Linux, and it will hold CRLF character-pair when running on Windows.

chomp

The chomp function uses the same Input record separator $/ to determine what to remove from the end of the string. In normal circumstances the default behaviour is to remove the trailing, os-specific new-line from the parameter of chomp. That's what we do in most of the cases.

Changing the Input record separator

We could actually change the value of $/. For example, by assigning the letter Q like this: $/ = 'Q';. Then every call to the read-line operator $row = <$fh> will read in all the characters up-to and including the first Q.

In that case, calling chomp would remove the Q character from the end of the string.

We could also assign longer strings to $/ and then that would be the input record separator.

Let's try this perl program:

use strict;
use warnings;
use 5.010;

$/ = 'perl';
open my $fh, '<', 'data.txt' or die;
while (my $row = <$fh>) {
	say $row;
	say '---';
	chomp $row;
	say $row;
	say '==========';
}

On this data.txt file:

What do you think about perl, and what about
some other language called perl.
Or maybe Java?

The output looks like this:

What do you think about perl
---
What do you think about
==========
, and what about
some other language called perl
---
, and what about
some other language called
==========
.
Or maybe Java?

---
.
Or maybe Java?

==========

We can observe how each call reads up-to and including the word perl and then how chomp removes the string perl.

chomp removes only one copy of $/

In the following example we have multiple copies of the word perl at the end of the string and we set the Input Record Separator to be $/ = 'perl';. The first call to chomp removed one occurrence of perl. The second call to chomp removed the other occurrence. Calling chomp again, when there were no more copies of perl at the end of the string, did not do any harm.

use strict;
use warnings;
use 5.010;

my $str = "helloperlperl";

$/ = 'perl';
say $str;        # helloperlperl
chomp $str;
say $str;        # helloperl
chomp $str;
say $str;        # hello
chomp $str;
say $str;        # hello

This would be the same behaviour if we left the default value (the os-aware new-line) in $/.

chomp on arrays

If we put the read-line operator in list context, for example by assigning it to an array, it will read all the "lines" into that array. Each line will become an element in the array. Of course we have to put the word "lines" in quotes, as we already know that separating the content of the file at the new-lines is "only" the default behaviour. By changing the Input Record Separator we can split the file at any substring.

use strict;
use warnings;
use 5.010;

$/ = 'perl';
open my $fh, '<', 'data.txt' or die;
my @rows = <$fh>;
chomp @rows;

Calling chomp and passing the whole array to it will result in the removal of the trailing new-line (Input Record Separator) from every element.

slurp mode

We use the slurp mode when we want to read the content of a file into a single scalar variable. In that case we assign undef to the Input record separator.

perldoc

perldoc perlvar (search for $INPUT_RECORD_SEPARATOR), and perldoc -f chomp might have more to say about the topic.