A discussion in the CB about the use of $. (input line number) led me to an improved understanding of this infrequently used special variable. Consider the following code (yes, yes, I know it's an awful inefficient way to do it):

use warnings;
use strict;

open testFile, '>', 'delme.txt';
print testFile <<TEST;
Test file line 1
Test file line 2
Test file line 3
TEST
close testFile;

open testFile, '<', 'delme.txt';
while (<DATA>) {
    my $current = $_;
    chomp $current;
    print "$.\n";
    
    while (<testFile>) {
        print "$_" if $. == $current;
    }

    seek (testFile, 0, 0);
    $. = 0;
}

close testFile;

__DATA__
3
1
2
[download]

which prints:

1
Test file line 3
2
Test file line 1
3
Test file line 2
[download]

Note that the contents of $. is set to the line number for the most recently accessed file handle and that a seek counts as an access. Not only that, but the variable can be set for the current file handle by assigning to $.. Consider:

use warnings;
use strict;

open testFile, '>', 'delme.txt';
print testFile <<TEST;
Test file line 1
Test file line 2
Test file line 3
TEST
close testFile;

open testFile, '<', 'delme.txt';
<DATA>;
print "DATA: $.\n";
<testFile>;
print "Test: $.\n";
<testFile>;
<testFile>;
print "Test: $.\n";
<DATA>;
print "DATA: $.\n";
seek testFile, 0, 0;
$. = 0;
print "Test: $.\n";
<DATA>;
print "DATA: $.\n";

close testFile;

__DATA__
3
1
2
5
[download]

which prints:

DATA: 1
Test: 1
Test: 3
DATA: 2
Test: 0
DATA: 3
[download]

Note that these tests were run using ActiveState Perl version 5.8.7 build 813. YMMV.

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