In Perl, how can I read an entire file into a string?

I would do it like this:

my $file = "index.html";
my $document = do {
    local $/ = undef;
    open my $fh, "<", $file
        or die "could not open $file: $!";
    <$fh>;
};

Note the use of the three-argument version of open. It is much safer than the old two- (or one-) argument versions. Also note the use of a lexical filehandle. Lexical filehandles are nicer than the old bareword variants, for many reasons. We are taking advantage of one of them here: they close when they go out of scope.

With File::Slurp:

use File::Slurp;
my $text = read_file('index.html');

Yes, even you can use CPAN.

Add:

 local $/;

before reading from the file handle. See How can I read in an entire file all at once?, or

$ perldoc -q "entire file"

See Variables related to filehandles in perldoc perlvar and perldoc -f local.

Incidentally, if you can put your script on the server, you can have all the modules you want. See How do I keep my own module/library directory?.

In addition, Path::Class::File allows you to slurp and spew.

Path::Tiny gives even more convenience methods such as slurp, slurp_raw, slurp_utf8 as well as their spew counterparts.