Seek and tell are what you need for moving output around a file

# Open existing file
open(FILE, "+<filename.txt") || or die "error: $!";
seek(FILE, 0, 2);         # seek to the end of the file
print FILE "On the end";  # Appended to the end of the file
seek (FILE, 0, 0);        # seek to the begining of the file
print File "At the begining";
my $point = tell(FILE);   # tell where you are in the file
# Here tell will be 15 becuase you are at the end of "At the begining"
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These are all in the perlfunc docs