Can someone explain what this 'shift' word is doing in this subroutine???

sub get_month_year {
      my $file = shift;  #PLEASE EXPLAIN WHAT SHIFT IS DOING?
      my @params = stat $file;
      my @dparams = localtime($params[9]);
      my $month = $dparams[4] + 1;
      my $year = $dparams[5] + 1900;

      return($month, $year);
  }
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