open FILEHANDLE, '/etc/hosts' or die $!;
my $string = do { local $/; <FILEHANDLE> };
print($string);

The output of this Perl code is roughly:

127.0.0.1       localhost

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

That is, the entire contents of /etc/hosts.

Q&A

The difficult part to understand is line 2. In conventional Perl, reading a file’s complete contents would use a while loop to read line by line:

open FILEHANDLE, '/etc/hosts' or die $!;
my $string;
while (<FILEHANDLE>) {
    $string .= $_;
}

Here’s a detailed explanation of line 2:

• do { <block> } is a code block in Perl. The function’s return value is the last statement in <block>.
• <FILEHANDLE> is the last statement in the above do { <block> }, thus the return value. In Perl, <FILEHANDLE>’s return value has two different results depending on context:

my $scalar = <FILEHANDLE>;    # When left side is scalar, returns single line of file
my @array  = <FILEHANDLE>;    # When left side is array, returns entire file (each line maps to array element)

• The $/ variable setting is key to this syntax sugar. $/ is the input record separator, which defaults to newline. That is, <FILEHANDLE>’s default behavior of returning single-line content in scalar context is determined by the $/ variable. And local $/ is equivalent to local $/ = undef
• Difference between local and my: my creates a new variable, while local temporarily changes a variable’s value (within scope)

Overall, this creates a temporary scope through do {}, changes the value of $/ within the scope, changes the separator for <FILEHANDLE> in scalar context (from \n to undef), achieving the goal of reading the entire file content.

References

• The difference between my and local https://www.perlmonks.org/?node_id=94007
• Perl Idioms Explained - my $string = do { local $/; }; https://www.perlmonks.org/?node_id=287647