Here is a shorter reproducing script. I wrote it before I decided to google for this problem... :D

#!/usr/bin/perl -w
use strict;
foreach (0..1) {
  my $scalar;
  open my $h, ">", \$scalar;
  print "End of $_\n";
}
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Similar to the OP, this generates a warning the second but not the first time through the loop.

But unlike the OP, it just fiddles with a my $h handle, not STDOUT or anything like that.

Similar to the OP, initializing with my $scalar = '' removes the warning, but isn't this a perl bug? Shouldn't $scalar be in the same state both times through the loop when it isn't initialized?