You'll be in for a nasty surprise if you try and do that :) Try this snippet:

    use strict;
    seek DATA, 0, 0;
    print while <DATA>;
    __DATA__
    This
    is
    a
    test
[download]

From playing with that, you'll see that the seek returns to the beginning of the file, not the beginning of DATA. To correct that, you'll want to tell where DATA starts:

    use strict;
    my $start = tell DATA;
    seek DATA, $start, 0;
    print while <DATA>;
    __DATA__
    This
    is
    a
    test
[download]

Update: Reading jmcnamara's response. Boy, do I feel stupid. However, I'm now tempted to launch into my don't name a filehandle DATA rant, since this is a source of confusion (as demonstrated above). I recall ranting about this before. Humph. Maybe I'm just embarrassed :)

Cheers,
Ovid

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You'll be in for a nasty surprise if you try and do that :)

He isn't reading from __DATA__ he is reading from a filehandle called DATA that he opened himself.

In which case there is no problem:

#!/usr/bin/perl -w

use strict;

open DATA, "+>output.txt" or die "Error message here: $!\n";

print DATA "hello, world\n";
seek(DATA, 0, 0);

while (<DATA>) {
    print $_;
}

close DATA;
[download]

--
John.