Odd (undocumented?) behavior of RAM file within a loop

Greetings. I am writing a program which processes some files within a loop, and I am using the ability (as of Perl 5.8) to open a filehandle on a scalar reference (which I recycle in the loop). Kindly consider this stripped-down trivial code example which illustrates my question:

#!/usr/bin/perl
use strict; use warnings;

foreach (1..3) {
warn "Pass #$_\n";
my $file_in_memory;
open(my $fh, '>', \$file_in_memory) or die "Oops - $!\n";
close $fh;
}

__END__


It works fine for the first pass. However, subsequent passes emit warnings:

Pass #1
Pass #2
Use of uninitialized value in open at /perl/blah line 7.
Pass #3
Use of uninitialized value in open at /perl/blah line 7.

(line 7 is the "open" statement). Sure enough, if I make this change to "initialize" the scalar:

my $file_in_memory = '';

then I have no problems (except I bust the second rule of chapter 4 in Dr. Damian's _PBP_ - yikes).

Although the problem is easy enough to work around, I wonder why it behaves this way (which I don't see documented). The really strange thing (to me) is that it doesn't complain on the first pass. Any ideas?

Thx!

--
David Filmer (http://DavidFilmer.com)
.

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