Programming Perl, 4th Edition

Magically Banishing $_

This curiously exotic underscore tie class^[152] is used to outlaw unlocalized uses of $_. Instead of pulling in the module with use, which invokes the class’s import method, this module should be loaded with no to call the seldom-used unimport method (see Chapter 11). The user says:

no underscore;

And then all uses of $_ as an unlocalized global raise an exception.

Here’s a little test suite for the module:

#!/usr/bin/perl
no underscore;
@tests = (
    "Assignment"  => sub { $_ = "Bad" },
    "Reading"     => sub { print },
    "Matching"    => sub { $x = /badness/ },
    "Chop"        => sub { chop },
    "Filetest"    => sub { –x },
    "Nesting"     => sub { for (1..3) { print } },
);

while ( ($name, $code) = splice(@tests, 0, 2) ) {
    print "Testing $name: ";
    eval { &$code };
    print $@ ? "detected" : " missed!";
    print "\n";
}

which prints out the following:

Testing Assignment: detected
Testing Reading: detected
Testing Matching: detected
Testing Chop: detected
Testing Filetest: detected
Testing Nesting: 123 missed!

The last one was “missed” because it was properly localized by the for loop and thus safe to access.

Here’s the curiously exotic underscore module itself. (Did we mention that it’s curiously exotic?) It works because tied magic is effectively hidden by a local. The module does the tie in its own initialization code so that a require also works:

package underscore; use warnings; use strict; use Carp (); our $VERSION = sprintf "%d.%02d", q$Revision: 0.1 $ =~ /(\d+)/g; sub TIESCALAR{ my ($pkg, $code, ...