Perl 6, in our time. ;) I decided to take advantage of the 'bool' overloading context, and "fix" index(), rindex(), and system().

#!/usr/bin/perl -wl

use TrueFalse;

BEGIN {
  *CORE::GLOBAL::index = sub {
    my $pos = (@_ == 3) ?
      CORE::index($_[0], $_[1], $_[2]) :
      CORE::index($_[0], $_[1]);
    boolean $pos => ($pos == -1 ? false : true);
  };

  *CORE::GLOBAL::rindex = sub {
    my $pos = (@_ == 3) ?
      CORE::rindex($_[0], $_[1], $_[2]) :
      CORE::rindex($_[0], $_[1]);
    boolean $pos => ($pos == -1 ? false : true);
  };

  *CORE::GLOBAL::system = sub {
    my $ret = system(@_);
    boolean $ret => ($ret ? false : true);
  }
}  

print "j => $x" if $x = index "jeff", "j";
print "r => $x" if $x = rindex "jeff", "r";
system "ls" or warn "ls failed";
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The code behind this is rather simple -- it's just the "fixing" of the functions that looks tricky.

package TrueFalse;

use overload (
  '+0' => \&num,
  '""' => \&num,
  bool => \&bool,
  fallback => 1,
);

require Exporter;
@ISA = qw( Exporter );
@EXPORT = qw( boolean true false );

use constant false => 0;
use constant true => 1;

sub new { bless [ @_[1,2] ], $_[0] }
sub bool { $_[0][1] }
sub num { $_[0][0] }
sub boolean { TrueFalse->new(@_) }

1;
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japhy -- Perl and Regex Hacker