That will work, but as tilly pointed out, the problem is with the MergeSort code being named. This aught to work with the fewest changes to your code:

sub Merge(@) {
  my @array = @_;

  local $MergeSort = sub { ### Anonymous code ref. ###
    my $first = shift;
    my $last = shift;
    if ($last>$first) {
      my $mid = int(($last+$first)/2);
      $MergeSort->($first, $mid);
      $MergeSort->($mid+1, $last);
      my @b;
      
      @b = ( @array[$first..$mid], @array[reverse($mid+1..$last)] );
      my ($i, $j, $k) = (0, $last-$first, $first);
      for (; $k<=$last; $k++) {
        $array[$k] = ($b[$i]<$b[$j]) ? $b[$i++] : $b[$j--];
      }
    }
  }

  $MergeSort->(0, $#array);
  return @array;

}
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