If you also add use diagnostics; to your code, you would get more verbose help:

Use of uninitialized value in print at xxx line yyy, <STDIN> line 2 (#
+1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what ope
+ration
    you used the undefined value in.  Note, however, that perl optimiz
+es your
    program and the operation displayed in the warning may not necessa
+rily
    appear literally in your program.  For example, "that $foo" is
    usually optimized into "that " . $foo, and the warning will refer 
+to
    the concatenation (.) operator, even though there is no . in your
    program.
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The problem occurs on the following line:

print "$PID|$SID|",shift(@flds),"|$date\n";
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To debug this, I changed this line to:

print shift(@flds),"\n";
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and I still get the warnings messages. This leads me to believe that you are trying to shift an element from an empty array.