There's a charming example in the Perl Cookbook that ties global $_ so you get diagnostics when $_ is used globally (modified to be able to carp instead of croak if you want):
# croak on global underscore usage:
# no Underscore;
# carp on global underscore usage:
# no Underscore 'carp';
package Underscore;
use Carp ();
my $complain = \&Carp::croak;
sub TIESCALAR { bless \(my $dummy) => shift }
sub FETCH { $complain->("read access to \$_ forbidden") }
sub STORE { $complain->("write access to \$_ forbidden") }
sub unimport {
tie($_, __PACKAGE__);
$complain = \&Carp::carp if $_[1] eq 'carp';
}
sub import { untie $_ }
tie($_, __PACKAGE__) unless tied $_;
1;
And you save it as Underscore.pm and use it by just adding a
no Underscore; or
no Underscore 'croak'; . Or you can do it at the command line, of course, using
perl -M-Underscore myprog.pl or
perl -M-Underscore=carp myprog.pl