The first open opens c:\winnt\temp\hash.tmp for reading and for writing, but you didn't do a seek to the beginning of the file when you started reading.
Perhaps another wise monk could explain here, but I'm going to guess that since the file was never closed, its not reading an end of file marker and is just pushing out whatever is left in its stack.
You may have noticed the print statement never prints out to STDOUT

aba uba
niba seba
njak yuk

Add a seek and all may be well again:

#!/usr/bin/perl -w

open (TEMPO, "+>c:\\winnt\\temp\\hash.tmp");
print (TEMPO "aba uba\niba seba\njak yuk\n");

############open  (TEMPO, "<c:\\winnt\\temp\\hash.tmp");   

seek TEMPO, 0, 0;

while (<TEMPO>)                                 
{
 print;
}
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