#!/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";