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;
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--
John.