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.