I don't have Programming Perl, but I highly doubt that the book advised using that code to slurp a file into a variable, because it doesn't work. :) (Not that technical books are always correct, but this is rather spectacularly incorrect--not to mention incomplete.)

The main problem with this (aside from the fact that it's not complete) is that you've set the input record separator ($/) incorrectly. Setting it to "" tells Perl to read in paragraphs at a time, not the whole file.

To read in the entire file, set $/ to undef. So your sub should look something like this:

sub slurp {
    local $/ = undef;
    local *X;
    open X, $_[0] or die "Can't open $_[0]: $!";
    my $slurp = <X>;
    close X or die "Can't close $_[0]: $!";
    $slurp;
}
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Your other problem, which I've fixed, is that you were using $_, not $_[0].

The code should be used like this:

my $contents = slurp("/home/foo/bar.txt");
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The sub will die when it can't open or close the file, so you may want to change that; but you should always check for errors, no matter what.