#/usr/bin/perl
$tt = 1;
mt( \$tt );

sub mt {

  my $t = shift;
  print " sub: 1 t = ${$t}\n";
  ${ $t }++;
  print " sub: 2 t = ${$t}\n";

}

__END__
C:\test>junk55
 sub: 1 t = 1
 sub: 2 t = 2