how to understand shift?

[lina]

#!/usr/bin/perl

$month = "December";
$year = "2007" ;

header($month,$year);

sub header {

        my $month = shift ;
        my $year = shift ;  ######## here after shift the $year becomes null?
http://perldoc.perl.org/functions/shift.html

        print "-" x 79, "\n" ;

        print "$month $year Sales Report\n" ;

        print "-" x 79, "\n" ;

        $month = "January" ;
}

Thanks,

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