Reading a file's content into a variable

The manual way

open my $fh, '<', $filename
    or die "Could not open $filename for reading: $!";
my $contents = do { local $/; <$fh> };

After opening the file (read man perlio if you want to read specific file encodings instead of raw bytes), the trick is in the do block: <$fh>, the file handle in a diamond operator, returns a single record from the file. The “input record separator” variable $/ specifies what a “record” is—by default it is set to a newline character so “a record” means “a single line”. As $/ is a global variable, local does two things: it creates a temporary local copy of $/ that will vanish at the end of the block, and gives it the (non-)value undef (the “value” which Perl gives to uninitialized variables). When the input record separator has that (non-)value, the diamond operator will return the entire file. (It considers the entire file to be a single line.)

Using do, you can even get around manually opening a file. For repeated reading of files,

sub readfile { do { local(@ARGV,$/) = $_[0]; <> } }
my $content = readfile($filename);

can be used. Here, another global variable(@ARGV) is localized to simulate the same process used when starting a perl script with parameters. $/ is still undef, since the array in front of it “eats” all incoming arguments. Next, the diamond operator <> again delivers one record defined by $/ (the whole file) and returns from the do block, which in turn return from the sub.

The sub has no explicit error handling, which is bad practice! If an error occurs while reading the file, you will receive undef as return value, as opposed to an empty string from an empty file.

Another disadvantage of the last code is the fact that you cannot use PerlIO for different file encodings—you always get raw bytes.

Path::Tiny

Using the idiom from The Manual Way several times in a script soon gets tedious so you might want to try a module.

use Path::Tiny;
my $contents = path($filename)->slurp;

You can pass a binmode option if you need control over file encodings, line endings etc. - see man perlio:

my $contents = path($filename)->slurp( {binmode => ":encoding(UTF-8)"} );

Path::Tiny also has a lot of other functions for dealing with files so it may be a good choice.

File::Slurper

This is a minimalist module that only slurps files into variables, nothing else.

use File::Slurper 'read_text';
my $contents = read_text($filename);

read_text() takes two optional parameters to specify the file encoding and whether line endings should be translated between the unixish LF or DOSish CRLF standards:

my $contents = read_text($filename, 'UTF-8', 1);

File::Slurp

Don’t use it. Although it has been around for a long time and is still the module most programmers will suggest, it is broken and not likely to be fixed.

Slurping a file into an array variable

open(my $fh, '<', "/some/path") or die $!;
my @ary = <$fh>;

When evaluated in list context, the diamond operator returns a list consisting of all the lines in the file (in this case, assigning the result to an array supplies list context). The line terminator is retained, and can be removed by chomping:

chomp(@ary); #removes line terminators from all the array elements.

Slurp file in one-liner

Input record separator can be specified with -0 switch (zero, not capital O). It takes an octal or hexadecimal number as value. Any value 0400 or above will cause Perl to slurp files, but by convention, the value used for this purpose is 0777.

perl -0777 -e 'my $file = <>; print length($file)' input.txt

Going further with minimalism, specifying -n switch causes Perl to automatically read each line (in our case — the whole file) into variable $_.

perl -0777 -ne 'print length($_)' input.txt