After changing the names to protect the innocent, this is the script I'm using:

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

my $base_dir = "C:/Documents/perl";
while (my $line = <DATA>) {
    chomp $line;

    next if $line =~ /^\s*\#/;
#    next if not $line;

    my ($filename, $filepath) = split '\t', $line;

    if (not -d "$base_dir/$filepath") {
        system "c:/cygwin/bin/mkdir -p $base_dir/$filepath";
        print "Made $base_dir/$filepath\n";
    }

    if (not $filename) {
        warn "Filename is blank, filepath is [$filepath], line is [$li
+ne]\n";
    }
    else {
        system "mv $base_dir/$filename.xls $base_dir/$filepath";
        print "Moved $base_dir/$filename.xls to $base_dir/$filepath\n"
+;
    }
    print "\n";
}

__DATA__
filename1    some/file/path
filename2    some/other/file/path
filename3    yet/another/file/path/oooh/this/one/is/long
[download]

Made C:/Documents/perl/some/file/path
Moved C:/Documents/perl/filename1.xls to C:/Documents/perl/some/file/p
+ath

Use of uninitialized value in concatenation (.) or string at C:\Docume
+nts\perl\try.pl line 15, <DATA> line 2.
Use of uninitialized value in concatenation (.) or string at C:\Docume
+nts\perl\try.pl line 21, <DATA> line 2.
Filename is blank, filepath is [], line is []

Made C:/Documents/perl/some/other/file/path
Moved C:/Documents/perl/filename2.xls to C:/Documents/perl/some/other/
+file/path

Use of uninitialized value in concatenation (.) or string at C:\Docume
+nts\perl\try.pl line 15, <DATA> line 4.
Use of uninitialized value in concatenation (.) or string at C:\Docume
+nts\perl\try.pl line 21, <DATA> line 4.
Filename is blank, filepath is [], line is []

Made C:/Documents/perl/yet/another/file/path/oooh/this/one/is/long
Moved C:/Documents/perl/filename3.xls to C:/Documents/perl/yet/another
+/file/path/oooh/this/one/is/long
[download]

Note that I can easily fix this behavior by uncommenting the next if not $line part. The question is not how to avoid this error, the question is why it's happening in the first place.

All of the lines are read, and I don't see anything in the script that would cause me to read another line. I don't recall seeing this sort of behavior before, but I do normally include the check against empty lines.

What simple thing am I overlooking here? I SuperSearched for "__DATA__ null" as well as "__DATA__ empty" with no promising results. I've spent too much time trying to diagnose this problem, though, so I'm throwing in the towel and asking for ideas or information.

(by the way, "This is perl, v5.8.7 built for MSWin32-x86-multi-thread")

Update: I don't see the same behavior on my Mac (running OS X 10.4.1). "This is perl, v5.8.6 built for darwin-thread-multi-2level" So I'm totally baffled.