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;
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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