According to perldoc -f shift

    shift ARRAY
    shift   Shifts the first value of the array off and returns it,
            shortening the array by 1 and moving everything down. If t
+here
            are no elements in the array, returns the undefined value.
+ If
            ARRAY is omitted, shifts the @_ array within the lexical s
+cope
            of subroutines and formats, and the @ARGV array at file sc
+opes
            or within the lexical scopes established by the "eval ''",
            "BEGIN {}", "INIT {}", "CHECK {}", and "END {}" constructs
+.

            See also "unshift", "push", and "pop". "shift" and "unshif
+t" do
            the same thing to the left end of an array that "pop" and 
+"push"
            do to the right end.
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No mention of setting $_

TheStudent