unless ( eval " use Tk; " ) {
    &do_install ('Tk');
  }
  exec "my-real-program.plx";

# ...

sub do_install ($) {
  my $pkg = shift;
  print "Installing $pkg module...";
  if ($^O =~ /win/i) {
    `ppm install $pkg.PPD`;
  } else {
    chdir ('unix_pkg');
    `tar -zxf $pkg.tar.gz`;
    chdir ($pkg);
    require "Makefile.PL";
    `sudo gmake install`;
  }
}