Quite how you think that  ${$timer}++; is going to work when the local var is called $t and the global var $tt?

The first thing you need to do is get (back?) into the habit of enabling strict & warnings.

The (one) way to achieve what you want is to pass a reference to the variable:

#/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
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